style(Popover): adjust dark theme background color

src/popover/index.js

@@ -172,7 +172,7 @@ export default createComponent({
           ref="popover"
           value={this.value}
           style={this.location}
-          class={bem([this.theme, `placement-${this.placement}`])}
+          class={bem([this.theme])}
           overlay={this.overlay}
           position={null}
           transition="van-popover-zoom"

src/popover/index.less

@@ -184,8 +184,7 @@
 
   &--dark {
     color: @white;
-    background-color: @gray-8;
+    background-color: #4a4a4a;
-    opacity: 0.9;
 
     .van-popover__arrow {
       color: @gray-8;