I have been working on a number of client sites recently and there seems to be an every increasing need to use WordPress widgets, and why not as they are a great way of getting content where you need it. However styling them provided tricky as they all have similar classes (although this is flexible to a degree through register_sidebar(). Therefore I have developed the WP CSS Widget Classes plugin.
The plugin provides a select box on each widget that you add to a sidebar. This allows you to choose from the built in classes which are colour based. However you can always customise the classes in the select list by add a simple bit of code to your WordPress theme functions.php file. Maybe in a future release I will add a UI for this but for now code is needed.
The code is open sourced and available to browse, download, fork or submit a pull request from on Github. The repository is linked below: