jQuery UI uses a CSS Framework to help developers create custom jQuery widgets and plugins.
jQuery UI includes a robust CSS Framework designed for building custom jQuery widgets. The framework includes classes that cover a wide array of common user interface needs, and can be manipulated using jQuery UI ThemeRoller. By building your UI components using the jQuery UI CSS Framework, you will be adopting shared markup conventions and allowing for ease of code integration across the plugin community at large.
For more information about jQuery UI theming, go to the jQuery Learning Center.