This page details skinning information that applies to all skins.
For skin-specific customization, see:
How do I customize skins?Edit
Cascading Style Sheets (CSS) code can be put in several places to alter how your wiki looks.
Specific design details can be customized on a particular wiki by editing MediaWiki:Common.css, which affects all users of the site, no matter what skin they are using. CSS placed in MediaWiki:Common.css is usually aimed at customizing the display of the content area, such as defining customizations for templates, not the navigation or colors belonging to a skin.
On Wikia, multiple style sheets are combined in order to style what a user will see on any specific wiki. Wikia has its own stylesheets applied which create both Monaco or Monobook, depending which you are viewing. Any CSS customizations you make will be layered on top of these files.
Wikia's skin is named "Monaco", and MediaWiki:Monaco.css is the place where admins can add customizations. Admins can also customize stylesheets for Monobook by editing MediaWiki:Monobook.css, however very few users will actually see these customizations.
Can I customize my own view?Edit
If you'd like even more control than that, you customize your view (without changing the look for everyone) by editing your user stylesheet to add CSS code to change the site's fonts, colors, and many other things. Edit User:<yourname>/monaco.css to alter Monaco or edit User:<yourname>/monobook.css for Monobook. Creating skins through this way is a great way to be able to test and play around with the colors, yet not affect anybody on the site. You can then transfer the final code to a sit-ewide file of course.
These stylesheets which are specific just for you take the highest precedence over all the other files loaded. What this means is that your stylesheet takes priority and overrides the others if they conflict.