Backend a mess after upgrading to 2.9.1

[AliKK]

Hi,
I upgraded one of my Agent Theme sites yesterday, and the results were less than ideal. While the site looks good on the front end, the back end is all code. I contacted my host last night and they said the path was doubling up, which is why nothing would connect. They said they'd continue to work on it, and this is what they have sent me most recently:

I've cleared the warnings from displaying on the back end of your WordPress blog. However, the warnings are still occurring even though they do not show. This is because the theme "Agent" does not appear to be fully compatible with the latest version of WordPress. You may want to consider reverting your upgrade or finding a different theme that's compatible with the WordPress release you wish to use. If you need anything else, or if you have any questions or concerns, please don't hesitate to contact us again. Have a nice day.

I see there's a fix for the Single Post Template, but that doesn't seem to be my problem. Any clues?