It looks like antialiasing is off. Since you have mentioned it started occuring after you switched themes (I assume GTK), you could remove GTK settings for your user:
$ mv ~/.config/gtk-2.0 ~/.config/gtk-2.0.backup
$ mv ~/.config/gtk-2.0 ~/.config/gtk-3.0.backup
Then just re-login or reboot and it should use system's default settings for your themes.
If that does not help and you are not sure what else you have changed, a workaround would be to create a new user which would really get default settings again. You will, however, have to move your files and configurations of programs which you really need manually.