the above is a great solution, a simple one would be href="../CSS/Stylesheet.css"
I was using the same format
/Master
/CSS
If you do this you need to add the ../ to the file to backup one folder from the one you are currently in
Is there a reason why you are not using a Theme - In a theme you place both your skin file and css in the same directory under the APP_THEMES
/App_Themes/MainTheme
Then at the page directive or in web.config you simply state Theme="MainTheme" StyleSheetTheme="MainTheme"
http://www.asp.net/learn/videos/video-393.aspx this video shows how easy it is. If you need help - I can post more info for you.
This is also a simple way to have multiple themes and link the directly to each page you want wach theme to be.
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