A few things to start become Magento themes designer

There are few basic things that you should know already to be a Magento theme designer. Those are:

1. HTML
2. CSS
3. JS/jQuery
4. Some PHP

With the knowledge of above, you need to now know how Magento works.So 2 things to understand here.

1. Magento admin panel (This will give you an idea of adding your own theme from the configuration and other basic stuff.).
2. Now you need to understand the Magento theming principle. Fallback method, Layouts, templating.
3. Layout could be a difficult thing for starters but once you understand the gist you can easily carry on with it.

You can find lots of tutorial and explanation on above. if you understand above portions you can design your own custom theme.