When I go to https://ux.mendix.com/theme-viewer.html?profile=desktop?theme=silver_linings there is a theme switching option there. However this does not seem to be in the appstore (yet). You could try to raise a support ticket to see if Mendix would share this widget (and my presumption is that they used a widget for it. But may be somebody from Mendix can chime in here.).
Regards,
Ronald