I wish for this feature also, because currently there's not really a point of making vivaldi mobile app to follow the system theme. Before I go sleep I use dark mode, but only the Vivaldi UI changes to dark and the web pages are still in white, so the screen blinds my eyes. If I turn on "dark mode for web pages" in theme settings, all web pages turn to dark as default, so I have to turn on/off dark/white background manually from the menu. I hope for an option that dark mode automatically follow the system theme.
// actually I found the same topic with more votes and upvoted here also.
https://forum.vivaldi.net/topic/49255/dark-websites-schedule/29?page=2
hope this feature come soon