Warn multiple themes installation with activate

Author: ernilambar

src/Theme_Command.php

@@ -488,6 +488,9 @@ protected function filter_item_list( $items, $args ) {
 	 *     $ wp theme install http://s3.amazonaws.com/bucketname/my-theme.zip?AWSAccessKeyId=123&Expires=456&Signature=abcdef
 	 */
 	public function install( $args, $assoc_args ) {
+		if ( count( $args ) > 1 && Utils\get_flag_value( $assoc_args, 'activate', false ) ) {
+			WP_CLI::warning( 'Only a single theme can be active.' );
+		}
 
 		$theme_root = get_theme_root();
 		if ( $theme_root && ! is_dir( $theme_root ) ) {