How to run a complete GUI as root?

Read this before doing anything Why is it bad to log in as root?

First of all, you must set root 's password.

sudo passwd root
Enter new UNIX password:

Now you have root password. Now activate the root account:

sudo usermod -U root

Then you should allow lightdm, gdm or kdm to allow logging in as root.

To enable this:

In GDM

Edit /etc/gdm/custom.conf file and include AllowRoot=true.

Additionally, in newer versions of Ubuntu, you need to edit /etc/pam.d/gdm-password and comment out the line containing:

auth required pam_succeed_if.so user != root quiet_success

so that it looks like this:

#auth required pam_succeed_if.so user != root quiet_success

In KDM

Edit /etc/kde4/kdm/kdmrc file and change AllowRootLogin=false for AllowRootLogin=true.

In LightDM

This procedure enables the "Other" menu, so you can type the username root and login. You must edit /etc/lightdm/lightdm.conf and add greeter-show-manual-login=true. Then reboot.

Others session managers have other methods to accomplish this.

As you have noticed I gave the instructions but by any circumstance you should not run the GUI as root. Murphy's Law says that it is likely that you mess up somehow and will be annoyed starting again.

I played around enough, how do I go back?

To disable root login just type:

sudo passwd -dl root

Then revert the changes on the files above. Did it feel good having super-cow powers?

I've used nodm for this purpose on a LiveCD respin I had put together once upon a time.

sudo apt-get install nodm

Then configure it to enable and configure the user to use:

sudo vi /etc/default/nodm