Add a theme to Icon

Author: connor4312

docs/specification/draft/schema.mdx

@@ -319,6 +319,15 @@ export interface Icon {
    * If not provided, the client should assume that the icon can be used at any size.
    */
   sizes?: string[];
+
+  /**
+   * Optional specifier for the theme this icon is designed for. `light` indicates
+   * the icon is designed to be used with a light background, and `dark` indicates
+   * the icon is designed to be used with a dark background.
+   *
+   * If not provided, the client should assume the icon can be used with any theme.
+   */
+  theme?: 'light' | 'dark';
 }
 
 /**