Most probably the theme you've selected uses Unico engine, and you've been hit by this bug
If you're still able to open Cinnamon settings from Cinnamon's panel (though I doubt it because it's GTK+ 3 again), go there and change the theme to anything not using Unico (like Mint-X, for example).
If not, the best you can do is to remove Unico from the system. Switch into the system console by pressing Ctrl-Alt-F1, log in as root
and run this to remove the package:
- Code: Select all
apt-get remove --purge gtk3-engines-unico
Switch back to Cinnamon by pressing either Ctrl-Alt-F7 or Ctrl-Alt-F8 (usually the last one works).
If things are still broken, log out and log in back.