The file you want to edit on Ubuntu is:
~/.config/vivaldi/Local\ State
Specifically, you should remove:
"enabled_labs_experiments":["enable-force-dark@7"]
This is a nasty thing to stumble upon, as it completely breaks the browser from opening and there is not an obvious fix for the problem, except finding the file above and manually removing the change. Would be useful to have a "revert to working state" feature.