Welcome to the Plone community. We're happy you are here!
In answer to your question, yes the "Better Diff Viewer" project is about using HTML and CSS to make a better looking, more useful view for the differences between versions of content in Plone. There is another discussion happening with ideas about the Diff Viewer here in the forum. You should read that post to catch up on some questions and answers that are already here. And I would recommend you start your proposal in the summer of code website as soon as possible. The deadline is now 1 week away, so time is short!