Disappearing Custom Theme

Hey guys, I’m a newbie, but I’ve jumped right into theme development. I’ve spent hours trying to learn the basics, which was to be expected, but I’ve run into a really weird problem.

I was trying to go about creating my own theme via taking an existing theme and editing it to my desire. This was taking entirely too long, as my knowledge of CSS/XHTML is pretty rusty. So, I decided to create my own theme entirely.

So, the only thing I have uploaded to /themes/customtheme is a index.php, a style.css and an images directory with one .png. The index.php and style.css are straight from desk-mess, with the style’s header changed to reflect my information.

Editing goes smoothly, until I try and define a template theme for my theme. I believe this is what is causing the problem. When I try and do this, it suddenly brings up an error that says “Requested theme does not exist.” Suddenly, my theme has disappeared! It is not listed in the theme browser, nor can I edit it! The theme is still applied to my page however, I cannot edit it.

What the heck is up with this?

EDIT:
I just attempted to reupload my old style.css from the custom theme that I was working on (the aforementioned one). Seeing as it worked last night, I saw no problem with this. Wrong. It didn’t work, it gave me the same “Requested theme does not exist” error.
Great. I can’t edit anything that has anything custom in it. WHAT THE HELL?