browser detect then change css file if in ie6 or below .net

Hi guys, i currently have a css file, which is referenced from my config file, this works fine in firefox 2 and 3, google chrome, opera, safari and ie7, however when i view it in ie6 it looks awful, so i have set up a theme for ie6. Now what i need to do is have some piece of code that changes the line of code in the config file to the theme for ie6 if the users browser is ie6 or below, this bit of code can go into my masterpage file of course, does anyone have an idea of how to do this in .net, im programming in c#

Thanks