As you complete tasks and unlock new themed areas to expand your town, you’ll gain access to more elaborate building blocks, such as curved pieces. You’ll also have an expanding collection of “essences” - decorative objects that can be incorporated into your designs or used to paint things with pretty patterns.
While you can always enter houses and start moving furniture - to the delight or annoyance of the residents - you have to gain the trust of your Sims before you can customize their personal appearance. As an example of the game’s “blank canvas” style, one of the designers told us it’s possible to give the whole place a Wild West flavor by sticking saloon doors on the buildings and forcing everyone to wear cowboy outfits. Whether the Sims’ll appreciate this kind of thing is another matter.