See Add a little Color Back To GUI it changes the compact view icons though you can edit the fullscreen icons.
You can't theme Biab though it has been ask for many times, you are basically stuck with those grays.
Maybe they can use this: http://www.alphaskins.com/index.php

You can use a resource editor to change colors in the Delphi forms but that's a bit of messing around.

You can use ReaTrak to integrate Biab an the Biab Plugin to Reaper.