A wayland settings protocol to tell clients about themes (icons, cursors, widget themes), fonts details (family, hinting preferences) etc.  Just send all settings at connect time, send updates when a setting change.  Getting a little close to gconf here, but could be pretty simple:

     interface "settings":
       event int_value(string name, int value)
       event string_value(string name, string value)

but maybe it's better to just require that clients get that from somewhere else (gconf/dbus).