I have a new XPages app with Domino 9 but I don't see an option to have OneUI3 there. I got also the XPages Demo App 2.0 from OpenNTF where this option exists, however I could not find where does it come from. If I just copy the demo app xsp.properties source code to my application, it does not work. Do I have to do something special with my application to get this theme?
There is no predefined choice or option for that. Just edit the source of your XSP properties file OR create and use a theme. Extend the theme from the "oneuiv3.0.2" theme to get this working. Details here: http://mardou.dyndns.org/Privat/osnippets.nsf/snippet.xsp?documentId=96AB51B29A4F4369C1257B1D0064D05F