This will actually perform `||`s against `spritesmith-custom`, is a rather long line, and might be confusing to revisit. How about the following?

```js
var cssFormat = 'spritesmith-custom';

// If there custom template, use it
if (cssTemplate) {
  json2css.addMustacheTemplate...
} else {
// Otherwise, override the cssFormat and fallback to 'json'
  cssFormat = data.cssFormat || ... || 'json';
}

// Render the variables via json2css
var cssStr = ...
```