Problem after testing Fixer and Upfront

I am ver interested in using the new Upfront themes. I tried out the Fixer one using the theme tester you guys recommended. I was able to work with the theme and test it out just fine. Nice stuff but will require some planning on our end before using it.

The problem occurred when I turned the theme testing off. When I went back to work with my installed theme all my custom CSS was gone from the CSS Stylesheet Editor. I think I got it all back now but it would be nice to tell me to bak that up.