The same here. Opera with Numix (or any other dark one) system theme is unusable. Have to launch it with
env GTK2_RC_FILES=/usr/share/themes/Adwaita/gtk-2.0/gtkrc
to make texts in menu readable.
The program window generally behaves weird (maybe because of the compositing issue), e.g. it's impossible to take a region screenshot with the menu open.
On top of all these, strange thing happens with the red "O" logo when enabling sidebar:
http://en.zimagez.com/zimage/screenshot2017-05-2000-38-05.php
Overall, Linux version of the browser feels quite unpolished and not really ready to be labelled "stable".
P.S: Many years ago I was taking part in the discussion regarding UIQ support, which has been dropped eventually. Now it seems that Opera is not really bothered with Linux support either, for the same issues travel from build to build. I really love using the Opera Browser, but such attitude from devs is unpleasant.