There isn't. The only way to truly change the appearance is through some hacky stuff (where you can change the color user overlays, but this sacrifices codes being more mobile friendly depending on how you do it). BBCode is a bit like shorthand CSS in a way, but in order to change the scrollbar you'd have to override the default class of the scrollbar which BBCode currently in unable to do.
You can do what other do and do a hidden scrollbar, but sometimes people will overlook hidden scroll content.