fix btn-tool display bug in nested cards

build/scss/mixins/_cards.scss

@@ -41,6 +41,7 @@
   .bg-#{$name},
   .bg-gradient-#{$name},
   .card-#{$name}:not(.card-outline) {
+    > .card-header {
       .btn-tool {
         color: rgba(color-yiq($color), .8);
 
@@ -49,6 +50,7 @@
         }
       }
     }
+  }
 
   .card.bg-#{$name},
   .card.bg-gradient-#{$name} {