so we can add card-outline-color support for the card-footer.

Header and footer styles are basic for purpose of custom theming.
I'm with @rafalp. I definitely do not want more styling added to bootstrap. Simplicity is something I would like to begin pushing for.
We won't be making changes to support theming of the inner components. One thing I might do is make those background-colors rgba so they can blend easier with parent backgrounds, but that's about it.
I agree with @SuperWangCC that it would have been nice to have the header and footer take on the same colour scheme as the outline class applied.
However, adding "bg-* text-white" to the card-header or card-footer produces a good result.
For example:
<div class="card" card-outline-success>
<div class="card-header bg-sucess text-white">Header</div>
<div class="card-block">
Content
</div>
</div>`
Most helpful comment
We won't be making changes to support theming of the inner components. One thing I might do is make those background-colors
rgbaso they can blend easier with parent backgrounds, but that's about it.