@stardepp Inside the theme editor, next to the enable button. It can only be pressed while the theme is enabled.
It gives you a preview of the original, unedited theme, similar to when you download a theme from themes.vivaldi.net, and reverts itself to your edited version after 60 seconds if you don't click on "Update" button from the confirmation dialog.
@leo32345 Agree and I guess will be done in batches. Consider the team is not so big and converting browser parts from chromium to Vivaldi UI is a big task. +1 especially for PDF&print page as I really dislike how chromium show that (and Edge did a nice work on that and setting page).