Fix dark custom variant

[carloschida]

As per the tailwind v4.1 docs, the custom variant dark should be

@custom-variant dark (&:where(.dark, .dark *));

So that, along with the next-themes provider the classes get applied.

This may very well be a stylistic choice but I found it troubling that main didn't not change background or inherited it from the body.

[carloschida]

I noticed that this is a consequence of a mismatch in the custom variant selector between shadcn and TW.

I opened it for discussion over there: shadcn-ui/ui#8481