Free jQuery theme Plugins
All the Free jQuery Plugins about 'theme' are listed here.
An easy-to-use jQuery based stylesheet/theme switcher that allows the user to select an alternate stylesheet from a dropdown based theme picker and remembers the user selection using HTML5 local storage.
Blast.js is a very small jQuery based theme / skin / color scheme switcher that makes it easier to quickly switch between color schemes on your webpage.
ColorPanel is a jQuery theme switcher used to create a sticky color panel that allows the users to change the theme/skin of your webpage by switching the style sheets.