demo: https://codesandbox.io/s/async-sky-5xyhv

body doesn't have a lightblue background as expected

```jsx
import React from "react";
import ReactDOM from "react-dom";

import styled from 'styled-jss';

function App(props) {
return (

const StyledApp = styled(App)({
border: '1px solid red',
'@global': {
body: {
backgroundColor: 'lightblue'
}
}
})

const rootElement = document.getElementById("root");
ReactDOM.render(, rootElement);
```

styled-jss: 2.2.3