Card header color theming (#26573)

Define new variable for card header color

scss/_card.scss

@@ -68,6 +68,7 @@
 .card-header {
   padding: $card-spacer-y $card-spacer-x;
   margin-bottom: 0; // Removes the default margin-bottom of <hN>
+  color: $card-cap-color;
   background-color: $card-cap-bg;
   border-bottom: $card-border-width solid $card-border-color;
 

scss/_variables.scss

@@ -767,6 +767,7 @@ $card-border-radius:                $border-radius !default;
 $card-border-color:                 rgba($black, .125) !default;
 $card-inner-border-radius:          calc(#{$card-border-radius} - #{$card-border-width}) !default;
 $card-cap-bg:                       rgba($black, .03) !default;
+$card-cap-color:                    $body-color !default;
 $card-bg:                           $white !default;
 
 $card-img-overlay-padding:          1.25rem !default;