Do not edit the Twenty Twelve theme. It is the default WordPress theme and having access to an unedited version of the theme is vital when dealing with a range of site issues. First create a child theme for your changes. Or use a custom CSS plugin.
Okay, esmi, I already have some custom CSS written for the theme which I like and have saved. I’m thinking I should start from scratch. If I delete my current Twenty Twelve and reload it, will that give me a clean version of the theme, from which I can then create my child theme?