Changing external style sheets using the DOM

“Most of us are familiar with manipulating an element's inline style by accessing its ‘style’ object. There are times however, that you may find it necessary to escalate the issue, and talk directly with the boss upstairs that's the external style sheet on the page. Modifying an external style sheet (style or link tag) is extremely powerful, yet should be noted is less efficient and straightforward than going the route of dynamic inline styles.”

http://www.javascriptkit.com/dhtmltutors/externalcss.shtml