The MapViewerStyleSheet.css files in the Web Mapping Application reference a file that does not exist (blank.gif) and returns a 404 error when using Fiddler.
From Derek on 8\3\8 : Change the path in each CSS to ../../images/blank.gif--Either delete the reference to images/blank.gif in the MapViewerStylesheet.css file for the application theme used, or copy the blank.gif file from Inetpub\wwwroot\<application name>\images and pase it into Inetpub\wwwroot\<application name>\App_Themes\<theme name>\images