Well, this was an easy fix. I can't believe no one jumped in and pointed us to the solution that was right under our noses in Theme Editing 101 here:
https://forums.cockos.com/showthread.php?t=172341
I'd delete my post just to spare myself some public humiliation, but since I did make an honest attempt to find the answer before posting, I figure someone else might miss it, too. Essentially, all you have to do is open REAPER's built in theme editor by opening the Actions lists and finding:
'Theme Development: Show theme tweak/configuration window'.
It's all right there. Somehow, I just missed the obvious. I hope this helps someone else whose brain is exhausted from diving into theme editing, spending way too much time figuring out which images to edit and how the rtconfig.txt file works but not not enough time sleeping!