Go to vivaldi://experiments
Check "Allow CSS modifications"
Create a custom CSS directory
ie: /home/user/.config/vivaldi/custom
Go to vivaldi://settings/appearance/
Edit Custom UI modifications and set it to the custom CSS directory
Copy vivaldi.css to /home/user/.config/wal/templates/
Run pywal
Create link to custom CSS created by pywal:
ie: ln -s /home/user/.cache/wal/vivaldi.css /home/user/.config/vivaldi/custom/vivaldi.css
Go to vivaldi://restart
If you are not using pywal then you can copy vivaldi.css directly into the custom CSS directory.
If editing vivaldi.css directly it is easiest to use find and replace to change {color#} to the colors of your theme.
Also, remember to remove the additioinal { and } around the CSS properties.