The difficulty of making your own theme depends entirely on your skill level with Cascading Style Sheets. Our best advice if you are a novice with CSS is to dissect the existing theme style sheets, modifying values to see what they do and adding your own selectors. Example style sheets can be found here:
- Adium Theme
- Minima Theme
- See the Using the Themes that AOL has provided section for additional CSS examples.
If you're already a CSS pro, have a look at the CSS Reference for a list of the selectors the API applies to the elements it creates and have at it.