Yeah, it seems like a bug in the dark theme, or at least a lack of optimization.
However, it has been like that for a very long time... And I remember having already reported it more than a year ago, because it had been already a very long time before that issue was present 😛 --> VB-45000
In general, I know that Vivaldi has a tremendous number of tickets to deal with, and that they can't make immediately happy everybody.
But I must admit that it's kind of a shame that such small issues (one value to change in the CSS code) can not be treated more quickly. Difficult to be a small team with impacts on so many users! ^^
Speaking about dark theme and "quick win", an other simple CSS value update could also make the background of the panel icons transparent. Having a light grey square on every webpanel icons in the dark theme is quite inelegant. Same thing, sadly also reported long time ago.