But I’m having trouble. At the moment I have my theme as Digg3, and when I edit CSS I clear all code and use the ‘start from scratch and just use this’ option. I then past the desired code in, but it does not show up like it’s supposed to.
The Digg3 theme doesn’t have that functionality. You can’t use CSS to change the way a theme actually works, so my advice would be either to start from scratch in Sandbox or choose a theme that works more like the theme you’ve chosen to mimic.
CSS is theme specific. You cannot use the CSS from one theme on another without all sorts of problems, and the themes here at wordpress.com are not even the same as the like-named themes available for self-hosted wordpress blogs. They have been adapted.