Manual:$wgAllowUserCss

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

When enabled, users are able to make personalised customisations 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 $wgAllowUserJs allows/disallows user JavaScript.