Don't know exactly where to put this so I'll put it here.
In Firedragon (which built off of firefox) there is a dark pdf reader mode. I've used it on firefox for a while and it generally matches your system theme. However, it did not work with firedragon on garuda.
I did some digging and found that the privacy.resistFingerprinting was to blame.
I don't know if this is reproducible on others' machines, but if you are getting a blinding white pdf reader when opening pdfs in firefox you can set privacy.resistFingerprinting to false.