A tropical climate for most of the year means that all you need to bring are lightweight clothes for your holiday in Thailand. If you plan to spend time at a beach resort, then you might also want to bring shorts and swimming gear.
In theory, all you really need to bring is your passport, plane ticket, and some means of getting money. All the rest can be easily purchased while you are in Thailand.
I frequent many Thailand forums and lots of guys ask about dress codes. The only place I have ever seen a specific dress code is at the Thai temples. The basic code is to dress business casual. Dont wear shorts and tank tops to a temple. Ladies need to cover up even if it means throwing a sweater or light jacket over bare shoulders. To be safe dress like you were going to church.
For nightlife, you can wear anything from jeans or shorts to casual or dress slacks. Shirts can be polo, button down, T-shirts or tank tops. It doesnt really matter. If you are in Pattaya or Phuket, almost anything goes. Wear what you are comfortable in.
I live in Hawaii and see tourists in Waikiki wearing beach attire while shopping downtown and the girls in bikinis...