The dark theme would be much better with lighter text on the dark background. That is the main problem I have with it. The contrast between text and background is too low. Is it such a difficult thing to make the text light when the background is dark?
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.