It's not too bad once you get used to it. It's still a lot of "throw this color here, check results, looks shit, change color, rinse and repeat." QT theming is pretty similar.
I had just taken days to perfectly set up my homemade theme last distro, matching QT and GTK, only to find out I didn't like the distro. I gave up after that and just slapped Gruvbox Dark on everything.
When in doubt and the work to theme gets too much: Gruvbox, Dracula, Tomorrow/Tomorrow Night, or Solarized will cover just about everything.
I mean, this might be a bit more your fault in this case, but I agree with the sentiment.
They're always changing something about the CSS sheets, and I find it a pain to develop for, granted it's been a few years since I last touched it, and on a very hobbyist level at that. I quickly switched to Qt for that project. Now I use wxWidgets, which I guess just uses GTK, but I like that I don't have to directly deal with GTK.
I don’t think that’s the case here. This is Lutris, a GTK3 app. There shouldn’t be any GTK changes breaking themes here. It seems like OP’s theme is just broken.