I'm on an Ubuntu based system, so an apt-get Post-Invoke hook would do the job for me [1]. I haven't bothered though. I just manually run a bash script that appends my !important changes to the end of the file.

But apparently there's an official way to apply CSS mods using vivaldi://experiments/ and Settings > Appearance

[1] https://www.cyberciti.biz/faq/debian-ubuntu-linux-hook-a-scr...