I copied the theme folder "Default", named the new copy with a different name.
I then changed the top comment of my index.php to:
/* Template Name: blabla */
I then changed the top comment of my style.css to:
/* Theme Name: Bla bla Theme URI: Description: Bla bla Author: Bla bla Author URI: Template: blabla Version: 0.1 . Created by Bla bla . */
I then went to admin panel and went to themes. Clicked activate on my template 'Bla bla' and it shows a white screen with nothing on it.
Unfortunately can't see anything now. Even out of admin panel.