Using themed css files requires a header control on the page. (e.g. head runat="server" ).

A search with the title text yields thousands of results each with almost similar solutions:
a.) Put the

tag on the page.
b.) Set EnableTheming = "False" on page. But this in itself doesn't work. Another workaround I saw was set StylesheetTheme="" Theme="" on the page. Even this didn't do the trick.

What worked for me was the comment on MS Connect page: http://connect.microsoft.com/VisualStudio/feedback/details/102039/using-themed-css-files-requires-a-header-control-on-the-page-e-g-head-runat-server">

So I added a web.config in the folder where I had the page that was throwing an exception and insert

< pages theme="">

My page was using MS Virtual Earth and this error would block my custom code to be called from the Mapping.js file to display the push pins on the map.