To link a cascading style sheet to an ASP.NET Web page
In Design view, from Solution Explorer, drag the style sheet file (.css file) and drop it anywhere on the Web page.
—or—
In Source view, from Solution Explorer, drag the style sheet file (.css file) and drop it within the <head></head> tags on the Web page.
A new link element is inserted inside the <head> tags, which might look like the following code example.
<link href="MyStyles.css" rel="stylesheet" type="text/css" />