Board Thread:Support Requests - Designing Your Wiki/@comment-3032314-20130803053354/@comment-4018164-20130803084759

Wikia uses classes to specify a target image for icons. You can use Inspect Element to find classes for each icon.

Once you find it, add to your MediaWiki:Wikia.css a code that overrides the default code (on this case, it uses as teh search box icon):