Themed towns are towns modified or hacked by players to resemble other things such as a space station, or an ocean. They mainly cover the ground entirely with patterns to resemble an alternate ground, road or path used a lot in urban themed towns. To make a themed town, most players have to cut down all the trees and remove most flowers, thus practically destroying their town environment and giving it a 'Worst Town' rating. However; if the player does well in transforming their town, it may well make up for the loss of nature and flora.
Some more advanced players use texture hacks instead of designs so that they can not only modify the ground surface but edit the patterns on buildings and cliff edges as well as the bus and the Bus Stop (City Folk only). Some use a tool for Animal Crossing: Wild World called the Town Map Editor which allows them to place any item, building, or plant anywhere they want, to help the themed town look better. It also allows them to edit the Town Layout and create a canyon or edit the ramp, sometimes they even delete the river and ocean if it is in their way.
A Legend of Zelda Themed town.
When some players move buildings around they often fail by mounting building upon building, making all but the lowest building inaccessible. Some place buildings in the sea but can edit the out-of-bounds areas (e.g. the river, holes, roofs, etc.) so they can get to them.
A Candy Themed town
Simple non-hacked themed towns include most commonly Candy and Urban.