Thread:Project:Support desk/common.js & .css/reply

Those pages are located at MediaWiki:Common.css and MediaWiki:Common.js respectively (also at your MediaWiki installation). Usually they are empty. Edit them like a wiki page (you need admin status on your wiki), fill them with your CSS code or JavaScript code and save them. As examples you may take the respective pages on this wiki here or at English or German Wikipedia:
 * English
 * en:MediaWiki:Common.css
 * en:MediaWiki:Common.js
 * German
 * de:MediaWiki:Common.css
 * de:MediaWiki:Common.js