Used in a Twitter thread

after.js

// ...
let theme = document.documentElement.className;
toggleButtonIcon(theme);

themeToggleBtn.addEventListener('click', () => {
  theme = theme === 'light' ? 'dark' : 'light';

  toggleButtonIcon(theme);
});

before.js

// ...
const initialTheme = document.documentElement.className;
toggleButtonIcon(initialTheme);

themeToggleBtn.addEventListener('click', () => {
  const currentTheme = document.documentElement.className;
  document.documentElement.className =
              currentTheme === 'light' ? 'dark' : 'light';

  const newTheme = document.documentElement.className;
  toggleButtonIcon(newTheme);
})