I found this web site - 
http://linuxg.net/how-to-change-file-and-folder-permissions-with-chmod-part-1/ 
-  to learn a terminal command to change permission to a usr/share/ 
folder so I could install a pidgin theme folder in pidgin directory but, 
obviously, without knowing what I was doing, Mint 15 got broken with that.

I entered the following command:
sudo chmod -R u-x,g-x,o-rwx usr/share/

And now booting Mint will give me gibberish dialog windows in an empty 
black background.

Either Nemo or Cinnamon is not working correctly and I would like to 
restore it back to its original setup.

I'm guessing the way to fix this is enter a command restoring all 
permissions to default in "drop to root shell prompt" in recovery mode.

Thankfully, I have also Ubuntu 13 installed in the same desktop PC so I 
could write this message, because Mint 15 is totally unusable.

How do I fix this or where do I ask for help?

Thanks for reading and hope to hear from you soon.

Denise.