Theme Service

Overview

IThemeService controls the application's visual appearance: which theme is active (Dark or Light) and whether the UI uses the compact density. Assigning a theme applies the matching Avalonia ThemeVariant to Application.Current.RequestedThemeVariant and takes effect immediately; compact mode switches the Fluent theme's DensityStyle between Compact and Normal.

Both values are stored in ThemeServiceConfig (theme defaults to dark, compact to false) and restored on startup. The service exposes its state as reactive properties, so a view model can read the current value and react to changes through the same member.

Available Themes

The default ThemeService ships these two themes:

Id	Constant	Avalonia variant
`dark`	`ThemeService.DarkTheme`	`ThemeVariant.Dark`
`light`	`ThemeService.LightTheme`	`ThemeVariant.Light`

The list is hardcoded in that implementation — there is no DI hook to extend it.

Usage

Read or change the current theme through CurrentTheme.Value. Always assign one of the items from Themes — any other IThemeInfo implementation is rejected and applies nothing:

// Switch to the light theme
var light = themeService.Themes.First(x => x.Id == ThemeService.LightTheme);
themeService.CurrentTheme.Value = light;

// Turn on compact density
themeService.IsCompact.Value = true;

To react to changes, subscribe to the property. SynchronizedReactiveProperty<T> pushes its current value on subscribe, so use Skip(1) when you only care about subsequent changes:

themeService.CurrentTheme
    .Skip(1)
    .ObserveOnUIThreadDispatcher()
    .Subscribe(theme => { /* react to the new theme */ })
    .DisposeItWith(Disposable);

Registration

The service is part of the core services and takes no configuration:

services.RegisterThemeService();

In a design-time environment the registration substitutes NullThemeService.Instance instead.

API

IThemeService

Manages the application's theme and control density.

Property	Type	Description
`Themes`	`IEnumerable<IThemeInfo>`	All available themes.
`CurrentTheme`	`SynchronizedReactiveProperty<IThemeInfo>`	Gets the active theme selected from `Themes`
`IsCompact`	`SynchronizedReactiveProperty<bool>`	Whether compact density is on. Assigning it applies the density and persists it.

IThemeInfo

Represents a theme item.

Property	Type	Description
`Id`	`string`	Unique id of the theme (e.g. `dark`).
`Name`	`string`	Localized display name.

ThemeItem

Represents the default IThemeInfo implementation.

Property	Type	Description
`Theme`	`ThemeVariant`	Avalonia theme variant applied when the theme is selected.

Last modified: 17 July 2026