Style My Tweets adds and easy to use options page for styling the Twitter widget that comes with Jetpack by WordPress.com. There is a good chance this plugin will work with other twitter widgets as well.
Options to style the widget container
Options to style the widget header
Options to style the individual tweets
This plugin requires the ThemeKit plugin.
ThemeKit is a WordPress plugin that helps you supercharge your self-hosted WordPress themes and plugins by providing a uniform way to create options.
For more information on Themekit, go to: themekitwp.com
Download and info: http://wordpress.org/extend/plugins/style-my-tweets/