When using ASP.NET themes and you specify the global theme in web.config you can run into problems when you want to create a page that does not use the theme.
If you just remove the <asp:Content> elements the page will error and you will probably get this message:
Using themed css files requires a header control on the page. (e.g. <head runat="server" />).
You should be able to get round this by setting EnableTheming="false" in the page directive:
<%@ Page Language="C#" EnableTheming="false" %>
Unfortunately this by itself does not work and you need to clear the Stylesheet and Theme values as well:
<%@ Page Language="C#" EnableTheming="false" StylesheetTheme="" Theme="" %>
Now you should be able to create a page without using the global theme.