Fix .card-header to respect --tblr-card-cap-bg (#2626)

Co-authored-by: terminalchai <213856599+terminalchai@users.noreply.github.com>
Co-authored-by: codecalm <codecalm@gmail.com>

.changeset/fix-card-header-cap-bg.md

@@ -0,0 +1,5 @@
+---
+"@tabler/core": patch
+---
+
+Fixed `.card-header` background-color to respect the `--tblr-card-cap-bg` CSS variable instead of being silently overridden by a `background: transparent` shorthand.

core/scss/ui/_cards.scss

@@ -220,7 +220,7 @@ Stacked card
   color: inherit;
   display: flex;
   align-items: center;
-  background: transparent;
+  background-color: var(--#{$prefix}card-cap-bg);
 
   &:first-child {
     @include border-radius(var(--#{$prefix}card-border-radius) var(--#{$prefix}card-border-radius) 0 0);