I have modified the Adwaita Dark Green theme and stripped it down to the basic Adwaita theme used by Gramps. The only changes I have made are to the Tabs and how they display.
I have removed the previous theme and replaced it with this new theme. Anyone interested can download it from my GitHub.
It is designed to work with the Themes addon
At this time my changes do not apply to the Dark theme. I am not a dark theme user so it may be awhile before I get to changing those tabs.