We highly recommend you to use our intelligent tools (design->layout, design->theme settings) to customize your storefront, however, you can always change your theme file directly from here.
To do so, you should have some kind of knowledge about html and css. And remember, if your storefront is malformatted after modification, you can always rollback by click Revert buttton on top-right.
Each theme includes lots of files, but you don't need to go through all of them. Here lists the major files.