Custom theme: compliance for 3.5.1

Hello, I’ve been learning to code/theme since October, and have created my own theme from scratch to display my work: https://www.gregorygainsborough.com

Upgrading to 3.5.1 broke the site pretty bad: nothing displayed except the grey background of the sidebar (an image file) and body {background:#000}… so, tail between my legs, I went back to 3.4.2.

I have not yet found a ‘master list’ of what would be needed to make my theme compliant with 3.5.1 (relative to 3.4.2) … can anyone help me out with that? What do I need to do to weather the upgrade?