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