fix(overlay): modify overlay background style

src/packages/__VUE/overlay/index.scss

@@ -14,7 +14,7 @@
   left: 0;
   width: 100%;
   height: 100%;
-  background-color: $overlay-bg-color;
+  background: $overlay-bg-color;
 }
 
 .nut-overflow-hidden {