My towns end up themed even when I'm trying not to do it... My first town I didn't know anything about the game and it ended up being modern/suburban themed, asphalt paths and sidewalks with lots of parks and modern benches and lights and bridges and modern pwp's (cube sculpture, solar panels etc) and modern upgrades. I sadly never saw it to completion because I got stuck with a villager who wouldn't move that was integral to my plan and I ended up scrapping the town after 6 months or so of trying to get him out. I did start a theme town on purpose at one point, it was narnia themed and I kick myself all the time for not setting up a dream address before I stopped working on it (it was not finished either). I reset to get the perfect map for what felt like forever... The plaza had just enough space in front of it for my main house (lucy's/professor Kirke's house, the house the Pevensie children are holing up in where the wardrobe is) so that the town tree could also represent the tree in the garden that Digory Kirke buries the rings under that eventually gets made into the wardrobe. There was a perfect amount of space between the plaza and the non-beach cliff side of the map for a dense forest of cedars and holly bushes that I stuck a street lamp into, and I managed to plot reset Gruff into a house right by there too (for Tumnus), I had obtained ALL the dreamies for the theme that I had planned that all pertained in some way to the series (gruff, elvis, dora, diana, julian, phoebe, leonardo, wolfgang, rasher, and hans) and had them all saying appropriate things, I had the main house to full size and also Edmund's house to full size (the white witch's castle) and had begun to collect statues for it and fill it with ice and regal and princess stuff. I had made paths and was landscaping, but ended up feeling overwhelmed. I cam so far, I really wish I had a way to visit it by dream to see what I had achieved, but that was poor planning on my part
My current town has evolved into a zen forest, and my secondary town is going to be very bright and fairy tale based with all drab villagers. I think I'll probably do a european or american forest one OR a zoo type animals one with one of my current cycle towns, and then the other one I may finally do an all dog town as well since I've wanted to for a while. Doing a hard set theme always gets to be too much for me every time I try but Narnia got the closest I've managed to get ever.
I definitely agree that theme towns are the best to visit, and that having, if not a theme, then at least a plan for what you want your town to be like definitely helps when making them and deciding how you want to plot your villager houses and stuff.