JSDocs for JSON schmea for Theme Manifest

src/Umbraco.Web.UI.Client/libs/extensions-registry/theme.models.ts

@@ -1,8 +1,16 @@
 import type { ManifestWithLoader } from "./models";
 
-
 // TODO: make or find type for JS Module with default export: Would be nice to support css file directly.
+
+/**
+ * Theme manifest for styling the backoffice of Umbraco such as dark, high contrast etc
+ */
 export interface ManifestTheme extends ManifestWithLoader<string> {
 	type: 'theme';
+
+	/**
+	 * File location of the CSS file of the theme
+	 * @example 'themes/dark.theme.css'
+	 */
 	css?: string;
 }