One of the biggest annoyances in ASP.NET is the way it adds CSS files to your <head> element of your pages. It insists on adding them to the end, offering you no way to add your own custom styles that override the theme unless you go down the line and add !important to each element.
This is especially detrimental if you're trying to add IE-only stylesheets using conditional comments. Since the theme files are loaded after the IE stylesheet, all your custom styles will be overridden as if it was not present at all.
There are some hacks, from using ...