this works (tested)
```css
section[data-dialog] > header:first-child .icon.icon-back {
  background-image: url(/shared/style/headers/images/icons/close.png);
}
.skin-organic section[data-dialog] > header:first-child .icon.icon-back {
  background-image: url(/shared/style/headers/images/icons/organic/close.png);
}
```