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UI Styling / Theming Conventions and Guidelines


This document contains guidelines and tutorials for Stying and Theming the UI of the application.

The design team defines the visual appearance of the application in using Figma designs.

These design can evolve and change over time, requiring the UI of the app to be updated accordingly.

The Nexus Mods App uses Avalonia for its UI framework.
Avalonia has a CSS-like styling system, which is used to style the UI.

Avalonia Styling

Avalonia uses a cascading (stacking) style system

In other words, styles defined at the highest level of the application (the App.axaml file) will be be used everywhere in an application. But, they can still be 'overwritten' closer to a control (for example in a Window, or UserControl).

When a matching style is located, then the matched control's properties are altered according to the setters in the style.

The XAML for a style has two parts:

  • A 'selector' attribute
  • One or more setter elements

The selector value contains a string that uses the Avalonia UI style selector syntax. Each setter element identifies the property that will be changed by name, and the new value that will be substituted.


<Style Selector="selector syntax">
     <Setter Property="property name" Value="new value"/>

The Avalonia UI style selector syntax is analogous to that used by CSS (cascading style sheets).


This is an example of how a style is written and applied to a Control element, with a style class to help selection:

<Style Selector="TextBlock.H1">
    <Setter Property="FontSize" Value="24"/>
    <Setter Property="FontWeight" Value="Bold"/>
    <StackPanel Margin="20">
        <TextBlock Classes="H1" Text="Heading 1"/>

Style locations

Styles can be placed inside a Styles collection element on a Control, or on the Application.

The location of the Styles determines the scope of visibility of the styles that it contains.
If a style is added to the Application then it will apply globally.

Styles can also be defined in separate XAML files, and then imported into the Application, e.g.:

<!-- TextBlockStyles.xaml -->
<Styles xmlns=""

    <Style Selector="TextBlock.h1">
        <Setter Property="FontSize" Value="24"/>
        <Setter Property="FontWeight" Value="Bold"/>
<!-- App.axaml -->
<Application... >
        <StyleInclude Source="/TextBlockStyles.axaml"/>

You can also include styles from a another assembly (i.e. project, NuGet, DLL) by using the avares:// prefix:

<Application... >
        <StyleInclude Source="avares://MyApp.Shared/Styles/CommonAppStyles.axaml"/>

Pseudoclass selectors and ContentPresenter weirdness

When styling a control, you may notice that setting some properties using pseudoclasses doesn't work as expected.

<Style Selector="Button">
    <Setter Property="Background" Value="Red" />
<Style Selector="Button:pointerover">
    <Setter Property="Background" Value="Blue" />
You might expect the Button to be red by default and blue when pointer is over it.
In fact, only setter of first style will be applied, and second one will be ignored.

The reason is hidden in the Button's template as defined in the default Avalonia Fluent Theme (simplified):

<Style Selector="Button">
    <Setter Property="Background" Value="{DynamicResource ButtonBackground}"/>
    <Setter Property="Template">
            <ContentPresenter Name="PART_ContentPresenter"
                              Background="{TemplateBinding Background}"
                              Content="{TemplateBinding Content}"/>
<Style Selector="Button:pointerover /template/ ContentPresenter#PART_ContentPresenter">
    <Setter Property="Background" Value="{DynamicResource ButtonBackgroundPointerOver}" />
The actual background is rendered by a ContentPresenter, which in the default is bound to the Buttons Background property.

However in the pointer-over state the selector is directly applying the background to the ContentPresenter (Button:pointerover /template/ ContentPresenter#PART_ContentPresenter).

That's why when our setter was ignored in the previous code example. The corrected code should target content presenter directly as well:

<!-- Here #PART_ContentPresenter name selector is not necessary, but was added to have more specific style -->
<Style Selector="Button:pointerover /template/ ContentPresenter#PART_ContentPresenter">
    <Setter Property="Background" Value="Blue" />

Style Classes Composition

A control can have multiple style classes applied to it, this allows for composition of styles.

<Style Selector="Border.Rounded">
    <Setter Property="CornerRadius" Value="8" />

<Style Selector="Border.OutlineModerate">
    <Setter Property="BorderThickness" Value="1" />
    <Setter Property="BorderBrush" Value="{StaticResource StrokeTranslucentModerateBrush}" />

<Style Selector="Border.Mid">
    <Setter Property="Background" Value="{StaticResource SurfaceMidBrush}" />
<Border Classes="Rounded Mid OutlineModerate">
    <TextBlock Text="Hello World!"/>

This should be used to avoid defining many different combinations of appearances. It can also be used for a sort of inheritance of sorts e.g.:

<!-- Base Standard Button (only use with additional qualifiers)-->
<Style Selector="Button.Standard">
    <Setter Property="CornerRadius" Value="4" />
    <Setter Property="Height" Value="36" />

<!-- Standard Primary -->
<Style Selector="Button.Standard.Primary">
    <Setter Property="Background" Value="{DynamicResource PrimaryModerateBrush}" />

    <Style Selector="^:pointerover /template/ ContentPresenter#PART_ContentPresenter">
        <Setter Property="Background" Value="{DynamicResource PrimaryStrongBrush}" />

<!-- Standard Secondary -->
<Style Selector="Button.Standard.Secondary">
    <Setter Property="Background" Value="{DynamicResource PrimaryStrongBrush}" />

    <Style Selector="^:pointerover /template/ ContentPresenter#PART_ContentPresenter">
        <Setter Property="Background" Value="{DynamicResource PrimaryWeakBrush}" />

Styles can also be defined nested in parent Styles, e.g.:

<!-- Base Standard Button (only use with additional qualifiers)-->
<Style Selector="Button.Standard">
    <Setter Property="CornerRadius" Value="4"/>
    <Setter Property="Height" Value="36"/>

    <!-- Standard Primary -->
    <Style Selector="^.Primary">
        <Setter Property="Background" Value="{DynamicResource PrimaryModerateBrush}"/>

        <Style Selector="^:pointerover /template/ ContentPresenter#PART_ContentPresenter">
            <Setter Property="Background" Value="{DynamicResource PrimaryStrongBrush}"/>

    <!-- Standard Secondary -->
    <Style Selector="^.Secondary">
        <Setter Property="Background" Value="{DynamicResource PrimaryStrongBrush}"/>

        <Style Selector="^:pointerover /template/ ContentPresenter#PART_ContentPresenter">
            <Setter Property="Background" Value="{DynamicResource PrimaryWeakBrush}"/>
This can be useful to keep the code more organized. Usage of nested definitions should be preferred over defining multiple styles with the same prefix.

Avalonia ControlThemes

In addition to Styles, Avalonia also supports WPF like ControlThemes.

ControlThemes are resources used to define the default appearance of a control, and can be overridden by Styles.

As they are resources, they can have a key and be assigned to a control using the Theme property. ControlThemes also support single inheritance using the BasedOn property.


<ControlTheme x:Key="EllipseButton" TargetType="Button" BasedOn="{DynamicResource {x:Type Button}">
    <Setter Property="Background" Value="Blue"/>
    <Setter Property="Foreground" Value="Yellow"/>
    <Setter Property="Padding" Value="8"/>
    <Setter Property="Template">
                <Ellipse Fill="{TemplateBinding Background}"
                <ContentPresenter x:Name="PART_ContentPresenter"
                                  Content="{TemplateBinding Content}"
                                  Margin="{TemplateBinding Padding}"/>
The above example defines a ControlTheme for a Button, which is based on the default Button ControlTheme (defined in the Avalonia Fluent theme).

Styling the App

In the App, all the Styles are defined in a dedicated Theme project NexusMods.Themes.NexusFluentDark.

This is so that all the styles can be easily found and modified in one place.

The theme project makes use of the Avalonia Fluent Theme as a base, which provides a default look for all the Avalonia core controls, using ControlThemes.

For non core controls, such as DataGrid or TreeDataGrid, there are additional themes imported to provide a default look

Styling the Avalonia Fluent Theme

The App is able to customize the default look provided by the Fluent Theme by passing a custom FluentTheme.Palette of colours.

This is done in the main NexusFluentDarkTheme.axaml file, which is the entry point for the Theme project.

The palette determines the default colours for Foreground, Background, Accent, etc.

These cascade down to all the controls, meaning that Foreground and Background colours don't need to be set explicitly unless they change from the default.

NexusFluentDarkTheme.axaml is Styles file and is imported into the main application inside App.axaml,

Styles in Nexus Theme

The Theme uses mainly Avalonia Styles rather than ControlThemes, for reasons detailed in Styling Approach ADR.

Styles should be organized into separate files for each control type, e.g. TextBlockStyles.axaml, ButtonStyles.axaml, TextBoxStyles.axaml, etc. Some control types may have multiple files, e.g. StandardButtonStyles.axaml and RoundedButtontyles.axaml.

Each style file needs to be made visible at the top level of App.axaml, and this can be done by importing it in the StylesIndex.axaml file.

Each Style file should contain a Preview showcasing the various styles in use, e.g.:

        <WrapPanel Margin="10" Width="600">
            <Border Classes="Rounded OutlineStrong" Padding="16">
                <TextBlock Text="OUTLINE Strong" />
            <Border Classes="Rounded OutlineModerate" Padding="16">
                <TextBlock Text="OUTLINE Moderate" />


ControlThemes are only used for base building blocks that need to be referenced in other Styles

In particular, the TextBlock typography styles.

ControlThemes should be considered for UserControls, where the ContentTemplate can be defined directly in the ControlTheme.

For usage in the App, the ControlTheme should either be a default one for all instances of the control, or have a Style class setting the Theme property, to avoid a direct reference to the ControlTheme in the UI project.

In general, prefer using Styles over ControlThemes, to avoid introducing unnecessary complexity.


Resources are used to define colours, brushes, fonts, ControlThemes, and other values such as Opacity, etc.

The use of these resources should be limited to the Theme project, and not used directly in the UI projects, as that would require a reference to the Theme project.

Resources should be placed under the Resources folder, and either be placed in a dedicated file, or in the generic ThemeBaseResources.axaml file. ControlThemes should be placed in a dedicated file for each control type, under the Resources/Controls folder.

Each resource file should be made visible to the rest of the Theme project by importing it in the ResourcesIndex.axaml file. Some exceptions may apply, for example for resources that should not be visible to the entire Theme project but only to the following hierarchy level.

Resource Aliases

Resources can be aliased by declaring a new resource with a new Key, referencing the original resource.

Resources can be referenced by either using the StaticResource markup extension or the DynamicResource one.

  • StaticResource resolves the resource at compile time, while DynamicResource resolves it at runtime.
  • StaticResource should be preferred inside the Theme project for both minor performance benefits, and compile time checking of the resource existence.
  • DynamicResource should be used when the Value of the resource can change at runtime, which should be rare.
  • DynamicResource also allow referencing a Theme resource from outside the Theme project without requiring a direct Project dependency.

Occurrences of outside references to resources should be rare, as most resources define appearance properties, which should be set by Styles, which in turn should be defined in the Theme project.


<x:Double x:Key="Alpha100">1.00</x:Double>
<x:Double x:Key="Alpha95">0.95</x:Double>
<x:Double x:Key="Alpha90">0.90</x:Double>
<x:Double x:Key="Alpha80">0.80</x:Double>
<x:Double x:Key="Alpha70">0.70</x:Double>
<x:Double x:Key="Alpha60">0.60</x:Double>
<x:Double x:Key="Alpha50">0.50</x:Double>
<x:Double x:Key="Alpha40">0.40</x:Double>
<x:Double x:Key="Alpha30">0.30</x:Double>
<x:Double x:Key="Alpha20">0.20</x:Double>
<x:Double x:Key="Alpha10">0.10</x:Double>
<x:Double x:Key="Alpha5">0.05</x:Double>
<x:Double x:Key="Alpha0">0.00</x:Double>

<!-- Opacity levels -->
<StaticResource x:Key="OpacitySolid" ResourceKey="Alpha100"/>
<StaticResource x:Key="OpacityTransparent" ResourceKey="Alpha0"/>

<StaticResource x:Key="OpacityStrong" ResourceKey="Alpha70"/>
<StaticResource x:Key="OpacityModerate" ResourceKey="Alpha40"/>
<StaticResource x:Key="OpacitySubdued" ResourceKey="Alpha20"/>
<StaticResource x:Key="OpacityWeak" ResourceKey="Alpha10"/>

<!-- Disabled Opacity level -->
<StaticResource x:Key="OpacityDisabledElement" ResourceKey="OpacityModerate"/>
In the example above, the OpacitySolid resource is an alias for the Alpha100 resource. The rest of the Theme should use the OpacitySolid resource, and not the Alpha100, as the latter is an implementation detail.

Color System

The App's Nexus 'Next' theme uses a 3 level color system based on Fluent Design, with some modifications.

This system was newly introduced to the App with the creation of the Theme project.

  • The first level is the 'Primitive' later, (e.g. Red100, Red50, Green100, ...).
  • The second level is the 'Brand' palette of 'Semantic' colours, (e.g. BrandWarning90, BrandInfo50, ...).
  • The third level is the 'Element' palette of colors that should be used directly in the Styles of the UI Elements, (e.g. SurfaceNeutralMidBrush, PrimaryStrongBrush, ...).

The main idea is that of separating the names of the colours used in the Styles, from the actual values.

This way, if the design team decides to change the colour of all Information elements, this can be accomplished by changing the value of the Info colour, without having to change the name of the colour in all the Styles.

The Theme project should always prefer to user the third level of the colour system when possible.

And the second level only if necessary. Never the first level.


  • The colours are defined in the Resources/Palette/Colours folder, with separate files for each level .

Avalonia has both a Color type and a SolidColorBrush type.

Some properties require the former, while other require the latter.

Semantically, a color is just a value, while a brush could have a texture, or a gradient, etc. In practice, the difference is that SolidColorBrush also includes an Opacity property, which is not present in Color.

The brush version of a color should be preferred when both can be used.

The only place in the entire code where hex color literals should appear is in the PrimitiveColors.axaml file, everywhere else colors should be referenced through higher resource aliases.


Like the colors, the App has a system of opacity levels.

  • The primitives are defined in Resources/Palette/Colors/PrimitiveOpacities.axaml.
  • The 'element' (third) layer is defined in ThemeBaseResources.axaml.

Developers should avoid using numeric values directly and instead strive to use the semantic aliases instead.

In particular a OpacityDisabledElement alias is defined, which should be used for disabled elements.

Other Resource Palettes

Like Opacity and Colors, the App has alias palettes for other resources

Value aliases provide an abstraction over the actual values, and should be used in the Styles of the UI Elements.


A numbered alias system following the Spacing-none, Spacing-1, Spacing-2, ... pattern. The values are used to define the Spacing property of Panel controls such as StackPanel. The values can be found in Resources/Palette/Spacing/ElementSpacing.axaml and /Styles/Controls/StackPanel/StackPanelStyles.axaml contains classes to apply the spacing to the StackPanel control.


Resources/Palette/CornerRadiuses/ElementCornerRadius.axaml contains the aliases for the CornerRadius property, of controls such as Border. These follow the pattern Rounded-none, Rounded-sm, Rounded-md, ...

To round sides separately use or add an alias Rounded-{t|r|b|l}{-size}, e.g Rounded-t-lg for a large top rounding.

To round individual corners use or add an alias Rounded-{tl|tr|bl|br}{-size}, e.g Rounded-tl-lg for a large top-left rounding.

These aliases follow the pattern described on the Tailwind CSS documentation.


There's also a multi-level typography system for fonts and text styles.

The primitive fonts are defined in the Resources/Palette/Fonts/PrimitiveFonts.axaml file, and are then aliased in the SemanticFonts.axaml file.

<!-- Example ControlTheme using SemanticFonts -->
<ControlTheme x:Key="BodyMDNormalTheme" TargetType="TextBlock">
    <!--                       font family set via Semantic Font alias 👇 -->
    <Setter Property="FontFamily" Value="{StaticResource FontBodyRegular}" />
    <Setter Property="FontWeight" Value="Normal" />
    <Setter Property="FontSize" Value="14" />
    <Setter Property="LetterSpacing" Value="0" />
    <Setter Property="LineHeight" Value="21" />

TextBlockControlThemes.axaml defines all the Typography styles defined on Figma

These ControlThemes can then be applied by Styles, by setting the Theme property of a TextBlock to the name of the ControlTheme.

<!-- Kind of like this but it's done automatically for you! -->
<TextBlock Theme="{StaticResource BodyMDNormalTheme}" />

This way, a Style for a Button can set the Typography of the TextBlock inside it, without having to know the details of the Typography styles.

For each TextBlock ControlTheme an alias Style Class is defined in the TextBlockStyles.axaml file, to be used in the UI projects.

Example Heading2XLSemi class

<Style Selector="TextBlock.Heading2XLSemi">
    <Setter Property="Theme" Value="{StaticResource Heading2XLSemiTheme}" />


The app uses a custom UnifiedIcon control to display different types of icons.

This control supports Avalonia.Controls.Image, Avalonia.Svg.Skia.Svg, Avalonia.Controls.PathIcon and Projektanker.Icons.Avalonia.Icon. This permits the use of different types of icons interchangeably.

Adding & Placing New Icons

The app primarily uses Material Design Icons.

These need not be manually included in the project, as the App uses the Icons.Avalonia library, which offers a convenient way to use Material Design Icons in Avalonia.

To add a material design icon in the App, a new Style Class should be defined in the IconsStyles.axaml file. The class should set the Value property of the UnifiedIcon control to an IconValue, with the MdiValueSetter property set to the mdi-code of the icon.

<Style Selector="icons|UnifiedIcon.Close">
    <Setter Property="Value">
        <icons:IconValue MdiValueSetter="mdi-close"/>

The mdi code can be found by browsing for the icon on a site such as MaterialDesignIcons

The UI projects can then use this Style Class to set the icon without having to know the mdi-code.

<!--                 👇 -->
<icons:UnifiedIcon Classes="Close" />

This way all icons used in the app can easily be found in the IconsStyles.axaml file, and the mdi-code can be changed in one place if needed.

Scaling Icons

UnifiedIcons are assumed to be square, and should not have their Width and Height properties set.

The Size property should be used instead, which will scale the underlying icon to the specified size regardless of type.

Size="16" should be equivalent to Width="16" Height="16".

Using Styles in the UI

UI projects should change the appearance of controls by setting Style Classes on them

Appearance properties of a control in the UI should NEVER be set on the control themselves, but instead be defined in a separate Style.

Setting appearance properties directly on a control should be avoided because it will override any other styles that are applied to it, making the control appearance unchangeable. This includes changing appearance of states such as :pointerover or :pressed.

Developers should NOT define styles outside of the Theme project, e.g. in the UI projects.

Some exceptions may apply, for defining Control properties that determine the Layout of the UI, rather than the appearance.

These properties CAN be defined directly in the UI projects:

  • Padding
  • Margin
  • Spacing
  • Orientation
  • Alignment

Ideally these are defined in the top level <control>.Styles collection of the UI file, and not directly on the controls, but this is not a strict requirement.

Some functionality properties of controls can and should be defined directly on the control, e.g. Command, IsEnabled or Grid.Row, Grid.Column.

Naming Conventions

  • Style Classes should follow PascalCase naming convention.

  • ControlThemes names should follow PascalCase naming convention, and end with Theme, e.g. BodyMDRegularTheme.

  • Colors should start with the name of the palette level they belong to, e.g. SurfaceMid. SolidColorBrushes should end with Brush, e.g. SurfaceMidBrush.

3rd Party Theming

The app doesn't currently support switching themes or loading third party themes.

While there are no current plans for supporting theming, styles have been organized into theme projects to make this potentially easier to implement in the future.