Ok, I can now build Golly on my Mint 20 system with GTK3 (details are in docs/Build.html). I had to fix a few cosmetic problems with the spin ctrls in the Prefs dialog and with toolbar buttons (now borderless) so you might want to rebuild from the current repo to see how the changes look on your system. I've gone ahead and merged your changes to support dark mode, but they will only work when the app starts up. More changes would be needed to support switching to light/dark mode while the app is...
the Linux app uses desktop theme colors so the file panel should be readable in dark mode (thanks to Yutsing Kuh)
updated Golly version to 5.1b1
updated Lua to 5.5.0
more cosmetic changes for Linux app with GTK-3
simplified spin ctrls in Prefs dialog for building Linux app with GTK-3
updated Lua to 5.5.0 and Ag to 2.1
fixed problem with bitmap buttons in Linux app when using GTK-3