style: popup css bg

src/packages/popup/overlay.vue

@@ -1,6 +1,6 @@
 <template>
   <transition name="popup-fade">
-    <div v-show="show" class="bg nut-mask" :class="className"></div>
+    <div v-show="show" class="popup-bg nut-mask" :class="className"></div>
   </transition>
 </template>
 <script>

src/packages/popup/popup.scss

@@ -139,7 +139,7 @@ $popup-close-icon-margin: 16px;
 .popup-fade-leave-active {
   animation: 0.3s nut-fade-out;
 }
-.bg {
+.popup-bg {
   position: fixed;
   top: 0;
   left: 0;