The proper way to add extra external CSS files other then the default is to still use this:
mxui.dom.addCss(require.toUrl("WidgetName/widget/ui/WidgetName.css"));
The file 'WidgetName/widget/ui/WidgetName.cssi' , where WidgetName is replaced with the actual name of the widget, is automaticly combined in the widgets.css file that is created when deploying a Mendix application.
You should only use this if you really need extra external files. Another approach, since all is combined anyway, is to copy paste the CSS from the other files in to ''WidgetName/widget/ui/WidgetName.cssi'' ... Then you will get the same result in the end.