fix btn-tool display bug in nested cards

build/scss/mixins/_cards.scss

@@ -41,11 +41,13 @@
   .bg-#{$name},
   .bg-gradient-#{$name},
   .card-#{$name}:not(.card-outline) {
-    .btn-tool {
-      color: rgba(color-yiq($color), .8);
+    > .card-header {
+      .btn-tool {
+        color: rgba(color-yiq($color), .8);
 
-      &:hover {
-        color: color-yiq($color);
+        &:hover {
+          color: color-yiq($color);
+        }
       }
     }
   }