But I did see this comment further down the line. Reverting to the V6 or V5 themes might be worth doing.
I'm probably going to be cursed by three Scottish witches for telling this secret that shall only to be told among the chosen:
Reaper 7 runs absolutely fine with Reaper 6 themes and the Reaper 6 theme adjuster
The OP mentioned that he wants to adjust the Mixer Control Panel. In v5, you have some direct control of the mixer panel. It's under Layouts, plus you can make some changes by right clicking in the Mixer area. What specific changes are needed? The adjustments might be there. This would work especially for a 2 monitor layout where you could put the mixer on one monitor and have the wave files on the other.
I haven't upgraded from 5.99 but I downloaded V7 and put it on my XP machine just to putz around with it. Having used v5 for years, the changes to the controls was something that might keep me from upgrading, but the v5 and v6 themes are right there in the option menu. Switching to the v5 theme put me right at home.
I think I'll probably upgrade after Christmas, just to support the effort.