I've tweaked the (x)HTML and CSS so it gets as near to validating as it ever will. Tumblr use an iframe to insert the and , etc into the top right hand corner of the page. This throws a validation error. There's nothing I can do about that. And it's of no big concern.
The CSS is valid apart from the use of border-radius. Which is more an issue with the Jigsaw validation engine than an actual error.
All, in all, I'm quite impressed that Tumblr allows you to get syntax this good. It's a shame you can't use the HTML5 doctype with Tumblr, but again, not the end of the world.
Although almost valid, the mark-up still requires a fair bit of work. It's all a bit of a hack and could benefit from some tidying up. I'll do that in the meantime.