All my menus are highlighted like in the attached image.
I asked around and someone told me my gtk theme might be broken. However, I have no clue how to fix it. This occurs on my system menus, DE changer in the login menu, and most other programs.
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It's problem with unity-settings-daemon / gnome-settings-daemon. You can report a bug here.
On Ubuntu 14.04 and newer following command should fix problem (run it from a Terminal):
sudo killall -9 unity-settings-daemon && unity-settings-daemon & disown
On Ubuntu older than 14.04 tou can try (run it from a Terminal):
sudo killall -9 gnome-settings-daemon && gnome-settings-daemon & disown
In my case, for Unity I fixed this problem by deleting
~/.config/gtk-3.0 that I had left over from some earlier tweaking when still using 12.04.
With Xfce, I had this problem when using Settings / Appearance / ambiance-xfce-lxde theme that I got from http://www.ravefinity.com/p/ambiance-radiance-for-xfce-lxde.html
For Xfce, I fixed this by making a new theme with everything else from Ambiance, but the gtk-2.0 folder from ambiance-xfce-lxde:
cd /usr/share/themes sudo cp -r Ambiance MyAmbiance sudo cp -r ambiance-xfce-lxde/gtk-2.0/ MyAmbiance/
and then selecting MyAmbiance from Settings / Appearance.
Edit: Now on xubuntu-desktop on Ubuntu 16.10, and this trick still helps to fix some appearance issues when using the above theme.