of course it is a bug which will hopefully be fixed soon. the background color of the context menu seem to be the only problem in my mind. assuming it would be dark, following dark themes, the rest of the colors (selected,enabled, disabled items) would fit to the theme. i was not aware of which apps use gtk3 and which ones gtk2 therefore i was little be confused about opera using theme settings for gtk2.

i am on ubuntu 16.04 using unity an i can confirm that opera(45.0.2552.812) takes gtk2 theme settings instead of gtk3. i am not so much into gtk or themes at all but i worked around this by using gtk2 definitions in my dark theme (vivacious-unity-zfusion) from a corresponding light theme.

i replaced original "dark" definition in my themes gtk2 gtkrc file with:

style "dark"
{
bg[NORMAL] = "#DFDFDF"
bg[PRELIGHT] = shade (1.05, "#DFDFDF")
bg[SELECTED] = @selected_bg_color
bg[ACTIVE] = shade (0.95, "#DFDFDF")
bg[INSENSITIVE] = "#DFDFDF"
}