Will Opera 24 support GTK themes on Linux?
Older versions had a lot of problems with skinning on Linux especially on Ubuntu. The new version is looks much better but it still not theming the borders while Firefox and Chromium/Chrome already support this and it's looks a bit ugly with different themes: https://i.imgur.com/uPJwsfh.jpg
As Opera for Windows does not properly support Windows' themes, I doubt it ... they will try to make it look moderately close to native in several of the more popular desktops (though I understand the current release only has a Unity style so far), but they are trying to streamline things to have more speed. If support for theming takes too much away from rendering the page, then they'll skip it. (That's what their comments on themes in Windows come down to, so I'm sure it applies here as well.)