Add card-specific color variables

[tordans]

There should be an easy way to customize the color of the .card background.

Right now the colors are hard coded inside

bootstrap/scss/_card.scss

Lines 123 to 141 in 1a6d0cc

	//
	// Background variations
	//
	.card-primary {
	@include card-variant($brand-primary, $brand-primary);
	}
	.card-success {
	@include card-variant($brand-success, $brand-success);
	}
	.card-info {
	@include card-variant($brand-info, $brand-info);
	}
	.card-warning {
	@include card-variant($brand-warning, $brand-warning);
	}
	.card-danger {
	@include card-variant($brand-danger, $brand-danger);
	}

Instead there should be a default similar to this:

// Cards
$card-spacer-x:            1.25rem !default;
$card-spacer-y:            .75rem !default;
$card-border-width:        1px !default;
$card-border-radius:       $border-radius !default;
$card-border-color:        rgba(0,0,0,.125) !default;
$card-border-radius-inner: $card-border-radius !default;
$card-cap-bg:              #f5f5f5 !default;
$card-bg:                  #fff !default;
$card-primary-bg:          $brand-primary !default;
$card-success-bg:          $brand-success !default;
$card-info-bg:             $brand-info !default;
$card-warning-bg:          $brand-warning !default;
$card-danger-bg:           $brand-danger !default;

$card-link-hover-color:    #fff !default;

[tordans]

@cvrebert I can create a PR if you think this should be done.

[mdo]

Not happening with #22836—going to a Sass map based approach to generate these classes.