Manual:$wgAllowUserCss

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"

Details
This enables users to make a lot of neat personalised customisations (over and above the normal choice of skins within the 'preferences' display)

It may increase security risk to users and server load.

A related setting $wgAllowUserJs allows/disallows user javascript.

See also Manual:Skins