Rename gtkstyle.[ch] to pidginstyle.[ch] and fix up the dark mode check.

Testing Done:
* Ran with arc-dark and adwatia.
* Stepped through the debugger to make sure it was detected right.
* Joined a muc with both themes, the colors change but there're impossible to see as most of that code is elsewhere and not addressed in this review request.
* Also noticed that nicks in mucs are doubled, not sure what, but it's that way with out this patch as well.

Reviewed at https://reviews.imfreedom.org/r/208/