Clears the cache held by get_theme_roots() and WP_Theme.

function wp_clean_themes_cache( $clear_update_cache = true ) {
	if ( $clear_update_cache ) {
		delete_site_transient( 'update_themes' );
	}
	search_theme_directories( true );
	foreach ( wp_get_themes( array( 'errors' => null ) ) as $theme ) {
		$theme->cache_delete();
	}
}