Interesting question, Mark. They have always been depicted in shorts and a small shirt, as the boiler rooms were like...well..furnaces. As far as an "official" uniform, I have never heard or read about them having one. I personally always thought they just brought "stoking clothes" from home. However, I could be wrong.
They may have been depicted in shorts, but stokers of 1912 actually wore a lot more. They wanted protection against the direct heat from opened furnaces. They wore long trousers and their shirts were either short-sleeved or long-sleeved but with the arms rolled up. These, I think, were made of flannel. They wore strong boots and maybe a cap. Their only comfort was a sweat rag. This was either worn round the neck, or was stuffed into a pocket.
You can see a photo of a contemporary stoker at work on page 76 of Titanic Voices.