@FastTrackF1
Hm, the shortcut is much faster to me, you don't have even to look at the task bar.
Anyway, this is may possible with a custom modification, the different color for an active tab definitely is.
Check the modding forum section:
https://forum.vivaldi.net/category/52/modifications
Cheers, mib
EDIT: I couldn't find a feature request for this, you can create one.
https://forum.vivaldi.net/category/194/mail-calendar-feeds-feature-requests