There is a setting in opera://flags for that, but it has some other effects which you may not like. You can try it and turn it back off if the other effects bother you ...
We're about to enter 2019, and dark theme/skin is still the same, no improvement.
-right click menu
-in speed dial the folder are still in white...
Remaining items above still mismatch with dark theme which is possible to solve, I think so. Why dev don't really bother to do anything about that?
I face (almost) the same problem here. Things broke on version 49 because opera decided it can follow the theme's colors for fonts... and only for the fonts.
Debian testing x64, opera stable 49, openbox and numix as the theme.