Dispatching messages from web content in an add-on

background.js

const messageListener = port => message => {
  let theme;
  switch (message.type) {
    case "fetchTheme":
      log("fetchTheme");
      fetchTheme().then(({ theme: currentTheme }) =>
        port.postMessage({ type: "fetchedTheme", theme: currentTheme })
      );
      break;
    case "setTheme":
      theme = normalizeTheme(message.theme);
      log("setTheme", theme);
      storeTheme({ theme });
      applyTheme({ theme });
      break;
    case "ping":
      port.postMessage({ type: "pong" });
      break;
    default:
      log("unexpected message", message);
  }
};