Deal with deactivated theme white page

How do I get back into WordPress to change my deactivated theme?

The Community Site supports a wide array of WordPress themes for student blogs; however, sometimes certain themes may become outdated and need to be deleted due to technical and/or security concerns. If you receive a white screen stating that “The theme directory does not exist.” — Don’t panic.  The WordPress white screen of death is one of the most common WordPress errors. It is also perhaps one of the most frustrating because you are locked out of WordPress. No worries, the problem can be easily remedied by following a few basic steps… To get back into the WordPress dashboard to change a deactivated theme, simply use these instructions.

The white screen message should look something like this: