How to use the styled-jsx/server.flushToHTML function in styled-jsx

const appHtml = renderToString(
          
            
              
            
          
        )

        if (context.url) {
          response.redirect(context.url)
        } else {
          // Get Meta header tags
          const helmet = Helmet.renderStatic()

          const styles = flushToHTML()

          const html = view(APP_URL, NODE_ENV, helmet, appHtml, styles, initialState)

          // Reset the state on server
          store.dispatch({
            type: 'RESET'
          })

          // Finally send generated HTML with initial data to the client
          return response.status(status).send(html)
        }
      })
      .catch(error => {

);

const buttonColor = 'red';
const separatedCSS = css`button { color: ${buttonColor}; }`;
const withSeparatedCSS = (
    <div>
        <style>{separatedCSS}</style>
    </div>
);

const stylesChildren = flushToReact();
const jsxToRender = (
    { stylesChildren }
);

const stylesAsString: string = flushToHTML();
const html = `
    ${stylesAsString}
`;