Board Thread:Support Requests - Designing Your Wikia/@comment-28122555-20150111210126/@comment-6969786-20150119122103

You change the all aspecs of a wikia's overall look by using the Theme Designer

Special:ThemeDesigner

With the Theme Designer you can change the Logo, Set the Background Image, change the Favicon, set link color, background color and menu bar color among other theme related things.

However I believe you must be a Admin to access the Theme Designer.