There is a problem that broke my mind by 30 min. Then, I remember that all in linux is a file. 
The Problem:
On Linux Mint Petra
I have installed a new mdm login theme based on html. When I reboot the screen is black and a windows says: "There was a n error loading the theme -X-theme-".
Then, I click "OK" and the screen remains black and I couldn't log in. 
Solution:
1) open another instance terminal (CRTL+ALT+F1).
2) log in as root.
3) $ cd /usr/share/mdm/html-themes
4) $ ls (I saw the theme that I had installed and gives error with another 2 themes: "Clouds" and "Mint-X").
The point of this is to remenber a theme that was working fine.
5) $ cd /etc/mdm
6) $ nano mdm.conf (I want to edit mdm.conf file)
7) Once that is open to edit, search the line that is: HTMLTheme="name of theme" and modify to write the theme that you have on mind (that theme that works correctly). It should look smething like this:
# MDM Custom Configuration file.
#
# This file is the appropriate place for specifying your customizations to the
# MDM configuration. If you run mdmsetup, it will automatically edit this
# file for you and will cause the daemon and any running MDM GUI programs to
# automatically update with the new configuration. Not all configuration
# options are supported by mdmsetup, so to modify some values it may be
# necessary to modify this file directly by hand.
#
# This file overrides the default configuration settings. These settings
# are stored in the MDM System Defaults configuration file, which is found
# at the following location.
#
# /usr/share/mdm/defaults.conf.
#
# This file contains comments about the meaning of each configuration option,
# so is also a useful reference. Also refer to the documentation links at
# the end of this comment for further information. In short, to hand-edit
# this file, simply add or modify the key=value combination in the
# appropriate section in the template below this comment section.
#
# For example, if you want to specify a different value for the Enable key
# in the "[debug]" section of your MDM System Defaults configuration file,
# then add "Enable=true" in the "[debug]" section of this file. If the
# key already exists in this file, then simply modify it.
#
# Older versions of MDM used the "mdm.conf" file for configuration. If your
# system has an old mdm.conf file on the system, it will be used instead of
# this file - so changes made to this file will not take effect. Consider
# migrating your configuration to this file and removing the mdm.conf file.
#
# If you hand edit a MDM configuration file, you can run the following
# command and the MDM daemon will immediately reflect the change. Any
# running MDM GUI programs will also be notified to update with the new
# configuration.
#
# mdmflexiserver --command="UPDATE_CONFIG "
#
# e.g, the "Enable" key in the "[debug]" section would be "debug/Enable".
#
# You can also run mdm-restart or mdm-safe-restart to cause MDM to restart and
# re-read the new configuration settings. You can also restart MDM by sending
# a HUP or USR1 signal to the daemon. HUP behaves like mdm-restart and causes
# any user session started by MDM to exit immediately while USR1 behaves like
# mdm-safe-restart and will wait until all users log out before restarting MDM.
#
# For full reference documentation see the gnome help browser under
# GNOME|System category. You can also find the docs in HTML form on
# http://www.gnome.org/projects/mdm/
#
# NOTE: Lines that begin with "#" are considered comments.
#
# Have fun!
[daemon]
[security]
[xdmcp]
[gui]
[greeter]
HTMLTheme=Clouds (This is the line that you need to edit)
[chooser]
[debug]
# Note that to disable servers defined in the MDM System Defaults
# configuration file (such as 0=Standard, you must put a line in this file
# that says 0=inactive, as described in the Configuration section of the MDM
# documentation.
#
[servers]
# Also note, that if you redefine a [server-foo] section, then MDM will
# use the definition in this file, not the MDM System Defaults configuration
# file. It is currently not possible to disable a [server-foo] section
# defined in the MDM System Defaults configuration file.
#
8) Reboot your system. (you can use CRTL+ALT+DEL)
I hope that it could help you. Take a brake. 
