glamor-split-pane.js

let rule = css({
  '& .Resizer': {
    // ...
  },
  '& .SplitPane': {
    // ...
  }
  '& .Pane': {
    // ...
  }
})

const App = () =>
// add that rule to any parent element
  <div className={rule}>
    // SplitPane accepts a classname, so you could send a class here too
    <SplitPane className={css({ backgroundColor: 'gray', '& .Resizer': {...} })}>
    ...
    </SplitPane>
  </div>