Following the system one. Here's the fixed bug's ticket info - [Linux][Themes][Settings] “Settings → Themes → Theme Schedule → Operating System” does not work (VB-98462).
If you're up to installing the Snapshot and checking yourself, we'd appreciate it if you could confirm that it has been fixed for you as well.
https://vivaldi.com/blog/desktop/more-tab-position-fixes-vivaldi-browser-snapshot-4068-3/
