I foolishly lost the file I saved the CSS for Digg3 before I'd changed the background and tab colors, and so when I wanted to put it back, almost lost my marbles with frustration.
I found this old (closed) thread with the instructions from judyb12, and honestly... couldn't sleep tonight if I didn't say thank you.
The advice: "To restore the theme to it's pre-CSS-editing state, delete all the CSS out of the CSS editor (save it in a text file on your computer before doing so). Select "Add to existing stylesheet" and click "Save Changes".
Note: Selecting "Start from scratch" will ignore the theme's original stylesheet and only use what you put in the editing window."
I'm marking it resolved, but posting it to support in case anyone else is looking for the same answer.
Again... THANK YOU, THANK YOU!