Recently read that the dark theme got some improvements in the latest release.
I installed version 62.0.3331.18 hoping the problem of lack of distinction between active/non-active tabs had finally addressed.
Seems like it should be an important and easy fix. Yes, being able to identify which tab is currently active at a glance seems like an obvious thing to consider when designing anything that expects to be usable.
When you mouseover an inactive tab - that's a more obvious visual cue, and what should be considered for the active tab. As it is now (still), there is not near enough distinction between the active and non-active tabs. Not everyone's monitor can see the contrast.
If the theme utilized a user-editable style sheet to define the theme colours, everyone could be happy.
Until then, I will continue closing my eyes and randomly guessing what tab to click on. Yes, it's that bad.
Come on Opera, I'm still rooting for you after all these years.