[Improvement] Dialog: optimize style without content (#1233)

packages/vant-css/src/dialog.css

@@ -11,23 +11,17 @@
   border-radius: 4px;
   background-color: $white;
   transform: translate3d(-50%, -50%, 0);
-  
+
   &__header {
     padding: 15px 0 0;
     text-align: center;
   }
 
-  &__content {
-    &::after {
-      border-bottom-width: 1px;
-    }
-  }
-
   &__message {
     line-height: 1.5;
     padding: 15px 20px;
 
-    &--withtitle {
+    &--has-title {
       color: $gray-dark;
       font-size: 14px;
     }
@@ -46,6 +40,10 @@
     }
   }
 
+  &__header + &__footer {
+    margin-top: 15px;
+  }
+
   .van-button {
     border: 0;
   }