Edit underlying theme without visual editor

I’m trying to edit my theme (which is default to Casper), and most instructions refer to opening up some file, possibly with vim. However, I have only been able to access the visual editor and don’t know where to get started for finding the actual underlying code.

I would also like to make other edits to the blog, but it seems I only have the option for minimal code injection. What is the way to be able to have more control over the blog?