Hi Lydia,
You can pretty much wear what you want all of the time.
Shorts and T-Shirt are fine during the day, although if you're going to spend a while in the casino's you may want a sweater or light jacket because the air-con can get very cold!
Of an evening, you can wear the same. You can wear shorts/T-Shirts to shows, as many people do.
The only places that tend to have any dress codes are some top restaurants and lounges, and some of the clubs.
Cheers,
TC