You should be able to use MC on city hall and select Sim>Advanced>Trait>Change to add the hidden trait to every sims in your town. Just click the x when the filter is applied so all sims are selected. But I don't know a way to ensure, that every newly generated sim which comes via immigration or is born in game, will get this trait applied as well...
You could use NRaas Dresser to set a futuristic dresscode for your townies (
read here , scroll down to the valid parts section). This seems very much work in the beginning and I've never used it, but if you are really into the atmosphere, perhaps it is worth it?
I guess there is also a way of defining which cars are used by the townies and which cars are generated to give the feeling of traffic, but I have no experience with this, I would google NRaas+blacklisting vehicles and NRaas traffic if you want something like this.
I'm looking forward to other upcoming answers as I find this a really interesting topic