Board Thread:Support Requests - Designing Your Wikia/@comment-26437440-20160906021805/@comment-168424-20161014231255

Ok. You should always give as much detail and specifics as possible in a question so we don't have to guess stuff like this. You didn't mention JS in your initial question at all.

So, looking at w:c:popn-music:MediaWiki:Common.js where the button code is stored, it looks like CSS for the dark mode is at w:c:popn-music:MediaWiki:Darktheme.css.

Is that enough info to help you out?