It seems that Lingua Libre does not allow for user to use their own common.css and common.js. It would be useful to allow for people to be able to test modification before being applied to all.
User:{username}/Common.js :
mw:Manual:Interface/JavaScript: "If $wgAllowUserJs is set to true, users can customize the interface for only themselves by creating and importing personal scripts in certain user subpages."
User:{username}/Common.css :
Manual:CSS & $wgAllowUserCss. Set $wgAllowUserCss to True.