If you’d like to make a Tumblr theme, my recommendation would be three-fold (none of which necessarily involve photoshop by the way):

1 - Learn HTML and CSS. If you don’t know at least enough of these languages to be dangerous then you’re not going to get very far. The Tumblr theme engine is a beautifully simply thing, but it requires knowledge of the basics.

2 - Read the documentation. It’s very thorough, and it’s the best way to learn. Just have it open right next to your file and refer to it constantly.

3 - Look at someone else’s theme code to see how they put things together. There are a few different ways of making things work, and if you follow someone else’s basic pattern/framework and then change the look with CSS, you’ll save yourself the time of recreating the wheel.

Hope that helps!