hahahaha, this is a humorous misunderstanding...
Windowblinds is basically the only tenable solution to skin certain elements of the reaper UI, VERY SPECIFICALLY the exact window we are talking about hahahaha.
Its basically the only reason i have Windowblinds, which is a bit ridiculous on Reapers part, i love the program, have for more than a decade, but its really a bit insane they haven't updated this part of the UI in all that time...
For the record though I DID FIND A SOLUTION for the transparency! The issue is.... i cant remember what it was hahahahaha 
I went through my posts on their foru mand i cant find any posts i made about it, but i think i remember looking through a theme editor and untoggling something regarding that bar?
And finally, i do still have an issue using windowblinds in reaper, and other software. Its constantly flipping partway out of dark mode and so i have to reapply in order to see certain menus