Starting with the Winter ’06 release, the online application supports multiple user interface themes. Currently there are two user interface themes, “Salesforce” and “Salesforce Classic.” The getUserInfo() call returns a getUserInfoResult object, which includes the userUiSkin property. This property informs you of the user’s current user interface theme.
Stylesheets are available so partners can mimic the look and feel of the online user interface. For more information, see Styling Visualforce Pages in the Visualforce Developer's Guide.