Sorry, but the javascript can only manipulate DOM elements such as <img /> tags. It cannot dynamically replace CSS code.
That would be really sweet though!

Actually, it can override the included css file by specifying a new style for the given element at run-time.

• Ignored as a comment in other browsers - it’s only run by Windows IE.
• Works with (X)HTML Strict & Transitional Doctypes
• Does not break W3C validation
• Lightweight and very easily deployed
• Works with existing in-line or external CSS rules based on class or ID selectors
• Works with old-style img align="left" or align="right" attributes
• Runs after any existing Body Onload code
• Works with imagemaps and input images using a special version

• Doesn’t work in IE versions earlier than 5.5 - the AlphaImageLoader filter wasn’t introduced until version 5.5. There is no cure for earlier versions.
• Requires JavaScript to be enabled - the estimations for how many people choose not to use JS vary wildly. See note [1]
• CSS backround PNGs not supported - experimentation revealed I could traverse the Stylesheets collection and use the AlphaImageLoader trick to replace all background-image attributes containing PNGs, but then background-repeat would not work, and anchors placed over non-transparent parts of the image were not clickable :-(
• CSS rules for the PNG images based on the img selector are lost, but this is easily rectified by adding a class selector to your img rule such that img{..} becomes img,.png{..} - then add the attribute class="png" to each of your PNGs.