Card styles

This PR changes how we do card variants. Rather than generate custom classes for just the .card component for changing the color, background-color, and border-color, we're now using our utility classes. This provides more flexibility in how you ultimately style cards, while at the same time reducing our compiled CSS.

• Drops all card mixins. No more need for a .card-dark—which also coincidentally conflicted with .card-dark from the $theme-colors Sass map—and no more card-variant or card-outline-variant.

• Replaces all .card-{color} instances with a mix of .bg-{color} and .text-white as needed. For the outline ones, we're now using .border-{color}.

• Drops the .card-blockquote ahead of #22949 shipping, which removes most of the styling that we were overriding with that class. Might need some margin utilities still, though.

Docs have been updated, but might need to update the migration guide, too.