Manual:CSS

Cascading style sheet (CSS) markup sets much of the look and feel of MediaWiki -- font size, colors, spacing, the logo and background image, even whether stuff is displayed or is hidden.

To change the look and feel of MediaWiki's monitor and projector display (how it looks in a browser window) you can put CSS into MediaWiki:Common.css.

To change the way MediaWiki pages print, you can customize the CSS in the MediaWiki server file wiki/skins/common/commonPrint.css that for one thing by default hides the left column navigational stuff.

You can also create custom skins for MediaWiki.

Help
CSS syntax, attributes and values, must be correct or stuff won't work right. W3.org is an excellent reference for checking how to write CSS correctly:
 * http://www.w3.org/TR/CSS21/propidx.html -- CSS 2.1 Index of properties &larr; very helpful
 * http://www.w3.org/TR/CSS21/indexlist.html -- CSS 2.1 Index of everything
 * http://www.w3.org/TR/CSS21/cover.html#minitoc -- CSS 2.1 Table of Contents

Caveats
Some items should not be totally moved into CSS without double checking that they are still accessible to text browser users:

Table Borders
Table borders available via CSS are no substitute for still saying border="1" etc. See Help:Table. Otherwise text browser users will not be able to see the borders*.