My experience here (as I had to cope with this in com_localise), is that we miss a css:
when we use HTMLHelper::_('bootstrap.addSlide' we get the class class="card-header for the slide and class="accordion" for the <div>.
therefore we do need for example:

.accordion .card-header {
    background-color: #cfe6e9;
    something
    something else
}

No, the use of Bootstrap elements needs to be corrected. I don't know who decided to misuse the Card component's header class in incompatible contexts but that needs to be corrected, not furthered upon (if it's not wrapped in <div class="card"> then none of Bootstrap's card-* classes should be used, I'm looking at you module chromes...).

@mbabker I dont see any misuse of the card component here. It's purely a styling related component and has no JS associated with it, nor do the classes affect a11y.

It's simply due to the fact the background has been set to transparent:

https://github.com/joomla/joomla-cms/blob/4.0-dev/administrator/templates/atum/scss/_variables.scss#L139

Well then things have changed because there was a point where .card-header was being used with modules with no sort of wrapping .card anywhere. And to me that is misuse, even if it is "purely a styling related component".

Ahh in that aspect