Convert badges to CSS variables

[mdo]

No description provided.

[ffoodd]

--bs-badge-bg should be used indeed, but apart from that it looks good 👌

[ffoodd]

Regarding background, I wonder if we shouldn't reintroduce colors in badges, using CSS vars to only hydrate color and background-color (as we do in alerts already). That'd fix #34016 (which is requested several times already) without introducing any BC since existing usages based on utilities would still apply.

If not (or not yet) we may simply drop the useless CSS var and merge it almost as-is.

Any thought, @XhmikosR?

I just noticed a doc sentence to change regarding .bg-light and dark text: it also applies to warning and info theme colors. I'll take care of this when possible.

[mdo]

Tried getting the browserstack (push) to pass a few times, so giving up and merging anyway :).