Welcome to the madhouse!

You may find it easiest to change it from the dashboard instead of changing code if you're new at this.
Log in to site, Choose "Dashboard" tab on top right, select "Scrapbook" from the choices on the left column, and open Global scrapbook to edit the my_site_name_ (just erase the name from the editor and leave it blank) There's plenty of detailed documentation on this site to help with that.

The code you are looking for is in the Concrete file you downloaded. Open the file, choose the concrete file, select Themes, then Dark Chocolate, then Main. You'll need an editor to make and save changes (notepad in a pinch) and then upload the modified files to your site (same location path obviously)

If I can make a recommendation; Its pretty cheap to set up a "sandbox" or test site to try your mods on before committing them to your main website.

Just to clarify Adreco's response: you want to do either what he says in his first paragraph *or* what he says in his second paragraph (you don't need to do both).
As for the 3rd paragraph -- I respectfully disagree with this recommendation, as I don't think setting up a test site is a good idea for someone just starting out because it introduces too many complications.

Thanks for clarifying my answer Jordan ( I need to proof my own stuff better)
although.... I still really like my sandbox as a learning tool :P
lol