I was able to trim margins without problem, until I upgraded from KDE4 to Plasma5. It turns out I still had a lingering paper-colour setting that I was no longer aware of. Removing the line PaperColor=x,y,z from ~/.kde4/share/config/okularpartrc solved this for me. This issue manifests itself in Okular versions 0.20.3, 0.25.0 and 0.25.80. I use inverted colours a lot, which doesn't raise this problem.
I was able to trim margins without problem, until I upgraded from KDE4 to Plasma5. It turns out I still had a lingering paper-colour setting that I was no longer aware of. Removing the line PaperColor=x,y,z from ~/.kde4/ share/config/ okularpartrc solved this for me. This issue manifests itself in Okular versions 0.20.3, 0.25.0 and 0.25.80. I use inverted colours a lot, which doesn't raise this problem.