chore: switch样式优化

src/less/base/single/button/switch.less

@@ -18,4 +18,10 @@
   & .content-tip {
     color: @color-bi-font-switch-tip;
   }
-}
+}
+
+.bi-theme-dark {
+  .bi-switch {
+    background-color: @color-bi-background-switch-theme-dark;
+  }
+}