The widget you mention is not an Appstore widget, but a widget specifically build for certain projects. The widget is not supposed to be shared in the Appstore, since it has some quirks and constraints and is not applicable in general. I suggest filing any further bugs at support.mendix.com.
Regarding your problem, the files probably cannot be found since the filepaths have been moved between 2.5-rc2 and 2.5.1 (I think).
You need to either
adjust the paths to the images inside the CSS file