Can't log in after the update of your Linux Mint with Petra Backports ?
Don't forget to read the other tips:
Tip Of The Day (5) : Batch Script to resize pictures in linux (Nemo)
After the update i shot down my PC, and the following day i couldn't log in and got
"There was an error loading the theme XXXXXX"
And no way to log or anything
Here is the Workaround : 
- Press ctrl+alt+F1 to switch to the console
 - Log in
 - Then type sudo killall mdm
 - and startx
 - Cinnamon will start correctly (we've just bypassed the login window)
 
if not done yet, make sure mint-meta-cinnamon , mint-mdm-themes are installed, and ubuntu-system-adjustments is up-to-date.
==> you've got a somewhat working PC now, but you'll need to repeat the process at every boot
Now fixing the issue : 
When the update was performed it erased the potentially custom theme we were using so mdm couldn't launch anything at boot ...
- Once in Cinnamon trying to start mdm in order to change the theme fails.
 - Open /etc/mdm/mdm.conf as root
 - Search for HTMLTheme= and GraphicalTheme=
 - Change theses values to :
 
HTMLTheme=Mint-XGraphicalTheme=linuxmint
- Save the file, and reboot
 - Log in, open mdm and choose the theme you want (html or gdm/gtk)
 
==> You've now recovered a fully working Linux Mint 13 with the Petra backports ! 
Alternate fix [source] : 
- Delete your MDM configuration with the command: sudo rm -f /etc/mdm/mdm.conf
 - Reset the MDM configuration with the command: sudo touch /etc/mdm/mdm.conf
 - Reboot the computer
 
































