.. node-sass compatibility fixed (semi-colons)

sass/_mixins.scss

@@ -1,25 +1,31 @@
 @mixin card-variant($background, $color, $border) {
-    background-color: $background;
-    .card-footer, .card-header { border-color: $border; }
+  background-color: $background;
+  color: $color;
+
+  .card-footer, .card-header {
+    border-color: $border;
+  }
+
+  a {
     color: $color;
-    a { color: $color }
+  }
 }
 
 @mixin button-variant($color) {
     background-color: $color;
-};
+}
 
 @mixin alert-variant($color) {
     background-color: $color;
-};
+}
 
 @mixin progress-bar-variant($color) {
     background-color: $color;
-};
+}
 
 @mixin navbar-variant($color) {
     background-color: $color;
-};
+}
 
 @mixin icon-variant($color) {
     color: $color;