Are you asking for fuzzy transitions between themes or sharp transitions between themes, or both?
If this was aimed at me as the OP, then I guess both. What I can imagine myself doing is having a ride in my park with some theming on it, and then next to it another ride with different theming. Well how do you merge the two together so it doesn't look like a pirate ship on one side and (for example) a space ship on the other, in total conflict.
Just looking in community creations for example, one poster has done an amazing looking arachnid themed rollercoaster, but then next to the area he has built a mayan pyramid. Now, don't get me wrong,it looks nice - but isn't it odd having those two things next to each other? Or does it not matter? What if you were separating a kids ride area from a big rollercoaster?
From what I remember about a couple of parks I've visited in the uk, theming is patchy at best. Drayton Manor appears to have hardly any theming, with rides in the main area really just plonked down in the space available. Now Alton Towers (its been a long time since Ive been) had the Nemisis rollercoaster quite extensively themed, but that theming didn't (if I remember correctly) extend beyond that ride. Other rides were /are just plonked on the landscape. The old corkscrew rollercoaster looked like it had been assembled by gypsies on the local field using planks of wood and scaffold poles. However from what I remember about Alton Towers there is some quite large distances between some of the ride areas, which is mostly greenery, which I guess does separate the zones somewhat.
I've never been to a big USA park so I can't comment on those.
Maybe the issue for me is that when you look down on your park from above you can see all these things and they look conflicted, but of course as a visitor from ground level you can only see one thing at a time.