Can I use a 2.8 theme with 3.1 version of wordpress?

In theory, yes – though the results may vary from theme to theme.

I purchased a theme that I didn’t realize was 2.8 and put on a new installation. The site I’ve purchased it from has offered to “upgrade it” for another 49.00 – which I don’t have. It’s completely out of whack and while I’m well versed in HTML, not so much with css – I can modify existing where it’s logical – https://kaelinmarketing.com/ – I might be missing it, but is there someplace where I can read and learn about the differences that would be creating the broken theme so I can attempt to fix it myself?

Thanks in advance!