Adjust card head style property from background to background-color (#5374)

* fix color variable naming

* card head  background-color => background, this adds more flexibility for the mixin

components/card/style/index.less

@@ -22,7 +22,7 @@
   &-head {
     height: @card-head-height;
     line-height: @card-head-height;
-    background-color: @card-head-background;
+    background: @card-head-background;
     border-bottom: @border-width-base @border-style-base @border-color-split;
     padding: 0 24px;