Most jQuery developers out there use jQuery UI for their theming and UI interface components and I agree that it does the job pretty well. But, searching for themes for jQuery UI, I came across an alternative of it called Chico UI (an open source and free set of UI components that work perfectly with jQuery).
Like jQuery UI, Chico UI includes componets like auto-complete, blink, carousel, countdown, date-picker, dropdown, expand, calendar and many more. There is also a CSS layout framework called Chico Mesh for accomplishing web layouts and grids.
Besides this, Chico provides a CSS library for styling up HTML/UI elements like lists, tables, buttons, pagination, etc. Each Chico UI resource is highly documented and demos for each one is provided alongside with tutorials.