theme-switcher-react17.js

const themes = {
  localhost: () => import('../../styles/themes/default.js'),
  'client1.local.com': import('../../styles/themes/client1.js'),
  'client2.local.com': import('../../styles/themes/client2.js')
};

const Select = styled.select`
  position: absolute;
  bottom: 1em;
  left: 1em;
`;

const ThemePicker = ({ onChange }) => {
  return (
    <Select onChange={onChange}>
      {Object.keys(themes).map(key => {
        return (
          <option key={key} value={key}>
            {key}
          </option>
        );
      })}
    </Select>
  );
};

const ThemeImporter = ({ children }) => {
  const [theme, setTheme] = useState(null);
  const [domain, setDomain] = useState(document.domain);
  const previousDomain = useRef();

  useEffect(() => {
    if (previousDomain.current !== domain) {
      previousDomain.current = domain;
      (themes[domain] || themes.localhost)().then(nextTheme =>
        setTheme(nextTheme.default)
      );
    }
  });

  if (!theme) {
    return null;
  }

  return (
    <ThemeProvider theme={theme}>
      <div>
        {document.domain === 'localhost' && (
          <ThemePicker onChange={e => setDomain(e.target.value)} />
        )}
        {children}
      </div>
    </ThemeProvider>
  );
}

lol you mean React 16.7 ?