Hi Jeffrey,
You can check out this widget
https://github.com/rik-bos/StyleSheetSwitcher
Also the style sheet is linked on the index file. You could edit the html of the index file and link your external style sheet.
Maybe using mxui/dom which has "addCss" could enable you to do so:
https://apidocs.mendix.com/7/client/module-mxui_dom.html