fix(web): persist theme preference in settings

packages/client/ui-theme/src/theme-settings.ts

@@ -0,0 +1,22 @@
+/** Theme preferences stored in the Host user-settings document. */
+
+/** Settings namespace owned by the theme plugin. */
+export const THEME_SETTINGS_NAMESPACE = 'ui-theme'
+
+/** Field carrying the selected built-in theme preference. */
+export const THEME_PREFERENCE_FIELD = 'preference'
+
+/** Theme preference persisted by the product Appearance row. */
+export type ThemePreference = 'light' | 'dark' | 'system'
+
+/** Default preference when the user-settings document has no override. */
+export const DEFAULT_PREFERENCE: ThemePreference = 'system'
+
+/**
+ * Narrow one wire or registry value to a persistable preference.
+ * @param value - value crossing the settings or registry boundary.
+ * @returns whether the value is a built-in preference.
+ */
+export function isThemePreference(value: unknown): value is ThemePreference {
+  return value === 'light' || value === 'dark' || value === 'system'
+}