Avalloy.Themes

The native theme engine

Avalloy.Themes is the pure-Avalonia theming model: a set of hand-authored merged ResourceDictionary themes, one of which is swapped into Application.Resources at runtime while RequestedThemeVariant is set so Fluent's own control resources track light and dark. Controls bind tokens through DynamicResource, so a swap repaints live. Depends on Core only.

dotnet add package Avalloy.Themes

What ships

File	What
Themes/Tokens.axaml	Theme-independent tokens: fonts and their weights and features, sizes, radii, the tab-strip geometry. Merge it once in App.axaml.
Themes/Template.Light.axaml, Themes/Template.Dark.axaml	The two neutral palettes: every surface, edge, field, shadow, text tier and accent key. Copy one per brand theme.
Themes/Type.axaml	The type ramp as style classes: display, title, eyebrow, secondary, muted, caption, mono.
Themes/ControlStyles.axaml	The style vocabulary: card, actionbtn, ddrow, bare, and the rest.
Themes/AlloyControls.axaml	Control themes for Core's field controls and DrawerPull.

NativeThemeController

The IThemeController for this engine. Give it the catalog and the default ids, return it from ResolveThemeController, and it resolves the appearance model's light and dark slots and variant, System deferring to the OS, then swaps the dictionary and raises Applied.

private static readonly IReadOnlyList<ThemeDescriptor> ThemeCatalog =
[
    new("Light", "avares://Avalloy.Themes/Themes/Template.Light.axaml", Dark: false),
    new("Dark",  "avares://Avalloy.Themes/Themes/Template.Dark.axaml",  Dark: true),
    new("Ocean", "avares://MyApp/Themes/Ocean.axaml",                   Dark: true),
];

protected override IThemeController? ResolveThemeController() =>
    _theme = new NativeThemeController(this, ThemeCatalog, defaultLightId: "Light", defaultDarkId: "Dark");

protected override IAppearanceModel? GetThemeAppearance() => Prefs.Appearance;

ThemeDescriptor is an id, the avares:// URI of the dictionary, and whether it is dark. EffectiveThemeId(appearance) tells you which one is showing. To change theme, set the slot or variant on the appearance model and call ReapplyTheme().

Making it yours

Copy Template.Dark.axaml into your app, keep every key, and replace the values. That is the whole contract; the token page lists the keys. A theme that drops a key gets the Fluent default for whatever bound it, which is the failure mode to look for when a control comes up bare.

Converters holds the reusable value converters keyed off the contract, Accent, Surface.*, Border.*, Text.*, so any theme that ships those keys gets correct results.