Runtime theming

Runtime Theming

Instructions

Runtime theming means the same process serves a different theme without restart. Make it cheap to switch and cheap to render.

1. Model the Theme as Plain Data

A theme is an immutable data object. Switching means replacing the object and letting the framework recompose.

data class AppTheme(
    val colors: AppColors,
    val typography: AppTypography,
    val spacing: AppSpacing,
    val motion: AppMotion,
    val density: Density,
    val isDark: Boolean,
)

No mutable fields. No callbacks inside the theme object.

2. Expose Through Platform-Idiomatic Channels

Compose: one CompositionLocal per theme axis, read in components.

val LocalAppTheme = staticCompositionLocalOf<AppTheme> { error("AppTheme not provided") }

@Composable
fun AppThemeProvider(theme: AppTheme, content: @Composable () -> Unit) {
    CompositionLocalProvider(LocalAppTheme provides theme) {
        MaterialTheme(colorScheme = theme.colors.toColorScheme(),
                      typography  = theme.typography.toTypography()) { content() }
    }
}

SwiftUI: @Environment keys.

private struct DSThemeKey: EnvironmentKey { static let defaultValue: DSTheme = .light }
extension EnvironmentValues { var dsTheme: DSTheme { get { self[DSThemeKey.self] } set { self[DSThemeKey.self] = newValue } } }

RootView().environment(\.dsTheme, currentTheme)

Flutter: Theme.of(context) plus a ThemeExtension for custom tokens.

React Native: a React context with useMemo on the theme object; pair with a StyleSheet factory.

3. Persist the User Choice

Store only the choice, never the resolved theme.

enum class ThemePreference { System, Light, Dark }

class ThemeRepository(private val ds: DataStore<Preferences>) {
    val preference: Flow<ThemePreference> = ds.data.map {
        ThemePreference.valueOf(it[KEY] ?: ThemePreference.System.name)
    }
    suspend fun set(pref: ThemePreference) { ds.edit { it[KEY] = pref.name } }
    companion object { private val KEY = stringPreferencesKey("theme.pref") }
}

Resolve to the concrete theme at the root:

val pref by themeRepository.preference.collectAsState(initial = ThemePreference.System)
val isDark = when (pref) {
    ThemePreference.System -> isSystemInDarkTheme()
    ThemePreference.Dark   -> true
    ThemePreference.Light  -> false
}
AppThemeProvider(theme = if (isDark) DarkTheme else LightTheme) { AppRoot() }

4. Performance

• Keep the theme object referentially stable. New instance on every recomposition → everything re-reads.
• Split the theme by axis if one axis (e.g., density) changes more often than another (colors). Multiple small CompositionLocals beat one big one.
• Do not wrap the whole tree in an animated color transition on theme change; let the OS drive animation via the native transition (Compose Crossfade at the root is acceptable at 150–200ms).
• Never read theme fields inside tight loops (e.g., inside items of a lazy list in a draw phase); hoist to outer composition.

5. Animating the Transition

A 150–250ms crossfade at the root is enough. Avoid per-color lerps across the tree — expensive and often janky.

Crossfade(targetState = theme, animationSpec = tween(200)) { t ->
    AppThemeProvider(theme = t) { AppRoot() }
}

On iOS / SwiftUI, simply set preferredColorScheme and let UIKit drive the animation. On Flutter, AnimatedTheme is fine for light/dark; for brand switches, rebuild the root.

6. Density and Size-Class Switching

Density ("comfortable" vs "compact") and size class belong in the theme, not in per-screen state. Toggling density mid-session must not relayout the entire tree unstably — prefer a settings-gated restart if your layout is highly density-dependent.

7. Anti-Patterns

• Mutable singletons (ThemeManager.shared.theme = .dark) — invisible to frameworks.
• Recomputing the theme object on every render.
• Switching themes by replacing the root widget (MaterialApp) — loses navigation state.
• Persisting the entire theme blob to storage instead of the preference key.
• Per-component theme props; all theme access must flow through the provider.

Checklist

• The theme is an immutable data object, provided via the platform's canonical channel.
• User preference is persisted; concrete theme is resolved at the root from preference + system.
• Theme object identity is stable across unrelated recompositions.
• Switching theme does not reset navigation stack or lose input focus.
• A crossfade of 150–250ms at the root handles the transition; no per-color lerp.
• No mutable global theme state.