On the GRAMPS for Genealogists Facebook group, it is correctly noted:
one of the problems starting to “customize” GTK on Windows is that you are in risk of ruin other GTK software you use on the same machine…
I had to do a total reinstall of Windows 2 years ago because I started to tweak GTK themes, even with backups and restoring, two of the other software I used that happened to use the same GTK version as Gramps would not restore, I never managed to figure out why, so only way was to “restore” them, even after multiple uninstalls and clean ups of Gramps and all the other software I had installed that used GTK, was to reinstall Windows…
Actually it was a reset because I used hyper-v, but still… LOTS of problems… lucky for me, I didn’t do this on my main and “live” workstation where I have 4-5 other open source software using GTK.
Just a kind warning…
Since these are Cascading style sheets, perhaps our themes add-on could benefit from having a selection of a customized CSS sheet override(s) on top of the base theme?
That way our Gramps customizations would not affect other applications by directly modifying the shared base theme.
We could embed a link reference to a base CSS theme in the header of our customization overriding CSS file. Then it would inherit the stylings of the evolving base theme.
But that locks you into that particular base sheet. So it requires a manual edit if you want to explore other appearances without losing the hacks that compensate for GTK idiosyncrasies.