Go Green Theme renders improperly in IE 7, 8, 9

I’m trying to determine how to get the Go Green Theme to work properly in any “modern” version of Internet Explorer. If you visit https://www.shelbytrim.com/ in IE 7, 8, or 9, you can see how horrible it looks. I’ve added a plugin directing anyone visiting in IE to download Chrome or Firefox so that it renders properly, but of course, not everyone wants to do that.

I’ve tried adding the fix detailed here: https://ckon.wordpress.com/2009/03/19/how-to-fix-internet-explorer-8/ by appending that code to the end of the theme’s functions.php located in the wp-content/themes/the-go-green-theme folder, but that has not remedied the issue.

Any suggestions? Of course, please let me know if you need further information and I will be happy to supply it.