This updates `WP_REST_Themes_Controller::get_items()` to shortcut returning the current theme when the request explicitly queries for the active theme, avoiding expensive call to `wp_get_themes()`. Developed in https://github.com/WordPress/wordpress-develop/pull/11032 Follow up to r49925. Props aduth, mukesh27, westonruter. See #50152. Fixes #64719. Built from https://develop.svn.wordpress.org/trunk@61856 git-svn-id: http://core.svn.wordpress.org/trunk@61143 1a063a9b-81f0-0310-95a4-ce76da25c4cd