Hi there Random404,
You can easily add your homepage to the menu and it appear as your current_page_item but you don't need to touch any code!!
Are you developing your website on a child theme? Before you do anything else can I strongly recommend reading this article on why you should build your website using a child theme: http://themeshaper.com/thematic/guide/?page_id=66.
Assuming after you've read that you'll start using a child theme. Adjusting Thematic code directly is strongly advised against for so many reasons and should be avoided at all costs!!! All you need to do is go to your Wordpress Dashboard and navigate to:
1: Appearance > Menus
2: Then create your menu adding appropriate pages/categories etc,
MAKE SURE YOU GIVE YOUR MENU A NAME & CLICK "SAVE MENU".
3: Select your Primary Menu from the Dropdown (whatever you called it in step 2).
This can be found near the top-left hand side on the same screen. Again, MAKE
SURE YOU SAVE YOUR NEWLY CREATED MENU HERE.
4: To add your homepage see "Custom Links", copy and paste your homepage
URL and put this in the URL. Then enter "Home" or whatever you want to call it.
Then Click Add to Menu.
5: You can adjust the menu items order by dragging and dropping them to the
order you want.
You've added your homepage to your menu. The current_page_item styling should pick up when you're on it if you've got the CSS setup correctly.
I might add there are many brilliant people on here who may suggest other things but that's how I'd go about it!