Apparently, if you have Theme set across your web application, it's not enough to just set EnableTheming="false" in the @Page directive of your ASPX file to disable theming. You have to set Theme="" StyleSheetTheme="" too. Otherwise, if your aspx pretends it's something else (image, javascript, etc.) and has no <head> element, you'll get a nice exception like:
System.InvalidOperationException: Using themed css files requires a header control on the page. (e.g. <head runat="server" />).
