I found this great guide where Nick La specifically goes in detail to show you various aspects of creating a custom wordpress theme. It is really a great guide because it explain clearly with pictures how the results would be like and makes it very easy to learn to do a couple of these custom themes. 

Here is the excerpt:

“….In this tutorial, I will explain the basics of how WordPress theme works and show you how to convert a static HTML template into a theme. No PHP skill is required, but you need Photoshop and CSS skills to create your own design…”

You can read the full article here.