hugo-theme-hello-friend/source/js/theme.js

// Toggle theme

const getTheme = window.localStorage && window.localStorage.getItem('theme')
const themeToggle = document.querySelector('.theme-toggle')
const isDark = getTheme === 'dark' || getTheme === null

document.body.classList.toggle('dark-theme', isDark)

themeToggle.addEventListener('click', () => {
  document.body.classList.toggle('dark-theme')
  window.localStorage &&
    window.localStorage.setItem(
      'theme',
      document.body.classList.contains('dark-theme') ? 'dark' : 'light',
    )
})