Working with Atahualpa on my site

First post so here goes, I have been using wordpress for around 18 months and know my way around it but having recently discovered Atahualpa I want to create a new site. I know some Html although its been a while and CSS is zero but in order to start changing the theme around ie header images, colours layouts, etc what should I put some effort into understanding the basics of is it CSS or PHP or both ?
The forums great for individual problems but I want to be able to understand the basics of Atahualpa editing then try and solve most of the issues as they occur or revert to the forums for help if I cant. Any thoughts or comments ?