style(@vant/cli): improve mobile style in dark mode

packages/vant/src/button/demo/index.vue

@@ -144,8 +144,6 @@ const t = useTranslate({
 
 <style lang="less">
 .demo-button {
-  background: var(--van-background-2);
-
   .van-button {
     &--large {
       margin-bottom: var(--van-padding-md);

packages/vant/src/col/demo/index.vue

@@ -70,8 +70,6 @@ const t = useTranslate({
 
 <style lang="less">
 .demo-col {
-  background: var(--van-background-2);
-
   .van-doc-demo-block {
     padding: 0 var(--van-padding-md);
   }

packages/vant/src/empty/demo/index.vue

@@ -68,8 +68,6 @@ const active = ref('error');
 
 <style lang="less">
 .demo-empty {
-  background: var(--van-background-2);
-
   .custom-image {
     .van-empty__image {
       width: 90px;

packages/vant/src/loading/demo/index.vue

@@ -62,8 +62,6 @@ const t = useTranslate({
 
 <style lang="less">
 .demo-loading {
-  background: var(--van-background-2);
-
   .van-loading {
     display: inline-block;
     margin: 5px 0 5px 20px;

packages/vant/src/notice-bar/demo/index.vue

@@ -81,8 +81,6 @@ const t = useTranslate({
 
 <style lang="less">
 .demo-notice-bar {
-  background: var(--van-background-2);
-
   .van-notice-bar:not(:first-of-type) {
     margin-top: 4px;
   }

packages/vant/src/overlay/demo/index.vue

@@ -47,8 +47,6 @@ const showEmbedded = ref(false);
 
 <style lang="less">
 .demo-overlay {
-  background: var(--van-background-2);
-
   .wrapper {
     display: flex;
     align-items: center;

packages/vant/src/progress/demo/index.vue

@@ -72,8 +72,6 @@ const reduce = () => {
 
 <style lang="less">
 .demo-progress {
-  background: var(--van-background-2);
-
   .van-progress {
     margin: 20px 16px;
 

packages/vant/src/slider/demo/index.vue

@@ -96,7 +96,6 @@ const onChange = (value: string) => Toast(t('text') + value);
 
 <style lang="less">
 .demo-slider {
-  background: var(--van-background-2);
   user-select: none;
 
   .van-doc-demo-block {