first of all, check if this property is defined in custom preferences (you can add this here for example https://github.com/changm/gaia/blob/bug960325/build/applications-data.js#L220)

```js
let background_url = 'resources/images/backgrounds/default.png';
if (customize.background) {
   background_url = customize.background.url || background_url;
}
```

```js
background: {
  url: background_url
},
```