We've been given the option to create resource packs which can then be placed in the 'userpacks' folder, but this never works. In fact, no jars remapping any kind of graphic or sound placed in the userpacks folder will do anything. This means you have to update/repack the actual graphics.jar file. I've seen every resource pack to this date requesting that you replace those core files, instead of adding a separate pack in the userpacks folder.
I suspect this feature was never actually finished; has anyone else gotten user packs / resource packs to work without completely replacing the core graphics.jar etc. files?
This process https://forum.wurmonline.com/index.php?/topic/173260-wu-resource-packs-creation-and-usage/ doesn't work for me at all.
You need to select the beta branch of Wurm Unlimited on Steam via Right-Click -> Properties.