Say you use Trick-or-Treat on an Umbreon, which resists Ghost-type moves, then would its resistance and now weakness to ghost-type moves cancel each other out and make it take normal damage from Ghost-type moves? Same with taking normal damage from Dark-type moves and so on?
It adds the type "Ghost" to the target's current types. So yes, the Umbreon would take x1 effectiveness damage from a Ghost and Dark type move.
Source: the link you provided.