(I now realize I should’ve used the “reply as new topic” feature here. Apologies: its existence slipped my mind.)
In my experience this rarely works well — most of the users will be using the default scheme and those who don’t will likely be looking at a broken site, especially once you start piling on custom graphics, plugins and color tags.
[color=blue]Suppose you could choose between a light skin and a dark skin and picked the dark skin (like I’d do, probably!). You’d then read this paragraph and instantly regret it. Blue on black: eeeeek![/color]
There should be one, and only one, available style at any one time… by default.
(Small self-retort: you could fix this use case by making
color=blue set a
class="blue" style on a
span and then let each skin do what they will of that class. This wouldn’t be exactly compatible with all BBCode generators out there, but it’s not like there one canonical BBCode specification either.)