You could try and have a look at the css that is probably included in the widget.
TO do this seelct the widget in the widgets directory and rename the extension to zip and unzip the widget in a different directory. Look through the files to find the css definitions for the widget and see if this contains the cause.
If so fix the css and re-zip the complete content into the new widget making sure the rename afterwards to the widget extension. If fixed please inform the owner of the widget via github or mendix support as this is a mendix own widget.