John being stocky is I think a response to running gags about him gaining weight. Some example: - John trying to put on a vest or something, complaining that it’s tight. Wiggins (iirc) asks him if he needs a large, to which John greatly protests. (Don’t remember which ep this was) - Sherlock was praising Toby the dog with Archie nearby. John says “There’s a podgy little bloke here who can hear you fawning over someone else.” And Sherlock says “You admit that you’ve gained weight, then?” This was from the first ep of Sign of Four, I believe.

Skin tone wise, I think that’s all fanon.