Help:Extension:GlobalCssJs

The GlobalCssJs extension allow users to create a JavaScript page and a CSS page to be loaded in every wiki of a wiki farm. If you go to Special:Preferences > Special:Preferences on any wiki where the extension is enabled you'll find links to your global scripts and style sheets: On Wikimedia wikis, these global customizations are hosted on Meta-wiki.
 * meta:Special:MyPage/global.cssmeta:Special:MyPage/global.js

Variables
When adding scripts to your global.js, be aware that, as with gadgets, variables declared with " " are not attached to the  object: they are local variables whose scope is a wrapper function inserted by ResourceLoader to implement the global module (which is called " "). Therefore, if you plan to move a local script to the global module, and it needs to define global variables, make sure you use the syntax " " to declare them.

Per-skin customization
Currently the extension does not provide global CSS/JS for specific skins, but it is possible to customize the appearence of a specific skin globally by using classes such as "skin-vector" and "skin-monobook", which are added to the body element automatically by MediaWiki.