Hi.
It’s a pity that this theme doesn’t work for you. I have no clear idea why. The PC I was using in Cuba with darktable is configured with Fedora 37 and then I installed version 42 of the system, it always worked without problems of any kind. I was using it with Gnome. Now I have a laptop with Windows 11 that barely works, unfortunately it is not configured to be able to do much more than documents with Libre office, but I installed darktable the experimental version that uses AgX and I passed the .css in the configuration window. In fact, these last fixes I made to the theme I did them on this laptop and it looks without problems, the resolution is lower, because before I used a 32¨ 4K monitor and now the laptop is 15¨ and 1920x1080px but well, better times will come 
Here I leave screenshots of the fully functional theme installed on the laptop.
Greetings from Mérida, Yucatán.