[improvement] add padding variables (#3911)

src/overlay/demo/index.vue

@@ -4,7 +4,7 @@
       <van-button
         type="primary"
         :text="$t('showOverlay')"
-        style="margin-left: 15px;"
+        style="margin-left: 16px;"
         @click="show = true"
       />
 
@@ -34,41 +34,3 @@ export default {
   }
 };
 </script>
-
-<style lang="less">
-.demo-skeleton {
-  background-color: #fff;
-
-  .van-switch {
-    margin: 0 15px 10px;
-  }
-
-  .demo-preview {
-    display: flex;
-    padding: 0 15px;
-
-    .demo-content {
-      padding-top: 6px;
-
-      h3 {
-        margin: 0;
-        font-size: 18px;
-        line-height: 20px;
-      }
-
-      p {
-        margin: 13px 0 0;
-        font-size: 14px;
-        line-height: 20px;
-      }
-    }
-
-    img {
-      flex-shrink: 0;
-      width: 32px;
-      height: 32px;
-      margin-right: 15px;
-    }
-  }
-}
-</style>

src/overlay/test/__snapshots__/demo.spec.js.snap

@@ -2,7 +2,7 @@
 
 exports[`renders demo correctly 1`] = `
 <div>
-  <div><button class="van-button van-button--primary van-button--normal" style="margin-left: 15px;"><span class="van-button__text">显示遮罩层</span></button>
+  <div><button class="van-button van-button--primary van-button--normal" style="margin-left: 16px;"><span class="van-button__text">显示遮罩层</span></button>
     <div class="van-overlay" style="z-index: 1; display: none;" name="van-fade"></div>
   </div>
 </div>