Learning my way thru ASP.Net 4 basic web app-Collection of common programming errors

I am trying to learn asp.net 4 web app but playing around the web page template shipped in VS 2010.

I clicked New Project –> (Web) ASP.Net Web Application I click ok.

So now I am trying to manipulate the color of the blue header and the font color or the header text. But I am cant do it thru the css.

Any ideas what is going on in the background?

UPDATE
This is what I get in design time:

This is what I get at runtime: