changing CSS values on the fly

Hi,
 
To date, I've only worked with "hard-coded" CSS.  Recently, I've been looking 
for a means of dynamically changing CSS values on the fly... ie. create a link 
on the page, and when you click it, it runs some javascript that changes the 
CSS.  
 
Considering the number of items this would affect, using multiple CSS files and 
switching between them won't work.
 
Does anyone know how to change CSS values on the fly? ... any ideas?
 
Thanks!
Jeff




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