Theming a Vaadin application is not as frightening as some make it to be. It's just normal CSS with some Vaadin-related conventions, but not much more. In the end, you're using the same styles as with any HTML page.
To get you started, check out these helpful links!
- Theming in Book of Vaadin
- Creating a reusable Vaadin theme in Eclipse
- CSS basics & a bit more
- Vaadin Theme Tips and Tricks
- Themes in Vaadin Forum