@cavvs alright so just to make sure, I installed the opera gx browser and it didn't do the force dark thing properly either.
Then I tried installing Brave browser, and it turns out everything works just as expected in it when #enable-force-dark is enabled.
I think someone needs to look at opera's code and get it fixed at some point. I'm not coder, I wish I could help. So I'll leave it in the capable hands of the custodians of opera. Peace.