improvement: make card header margin configurable

src/scss/components/_cards.scss

@@ -7,6 +7,7 @@
 
 $card-bigpad: $lineheight;
 $card-smallpad: $lineheight_half;
+$header-margin: $lineheight;
 
 @mixin card($size) {
     @include border-radius();
@@ -20,9 +21,15 @@ $card-smallpad: $lineheight_half;
       margin-right: $size;
       &:first-child {
         margin-top: $size;
+        &.card-header {
+          margin-top:$header-margin;
+        }
       }
       &:last-child {
         margin-bottom: $size;
+        &.card-header {
+          margin-bottom:$header-margin;
+        }
       }
     }
 }