Manual:$wgAllowUserCss/en

Details
Enables user Cascading Style Sheets (CSS). When set to 'true', people can create their own User styles by creating a page called.

When enabled, users are able to make personalized customizations over and above the normal choice of skins within the 'preferences' display.

It may increase security risk to users, as well as incurring usually negligible additional server load.

A related setting  allows/disallows user JavaScript.