toggle-theme-after.js

let theme = document.documentElement.className;
toggleButton(theme);

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

  toggleButton(theme);
  localStorage.setItem(COLOR_MODE_STORAGE_KEY, theme)
})

toggle-theme-before.js

const initialTheme = document.documentElement.className;
toggleButton(initialTheme);

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

  const newTheme = document.documentElement.className;
  toggleButton(newTheme);
  localStorage.setItem(COLOR_MODE_STORAGE_KEY, newTheme)
})