webbed-site/theme.js

const storedStyle = localStorage.getItem('style');
if (!storedStyle) {
  setTheme('green');
} else {
  setTheme(storedStyle);
}

const switches = document.getElementsByClassName('switch');
for (const el of switches) {
  el.addEventListener('click', () => {
    setTheme(el.dataset.theme);
  });
}

function setTheme(theme) {
  if (theme === 'peach') {
    document.getElementById('switcher-id').href = '/themes/peach.css';
  } else if (theme === 'moon') {
    document.getElementById('switcher-id').href = '/themes/moon.css';
  } else if (theme === 'sakura') {
    document.getElementById('switcher-id').href = '/themes/sakura.css';
  } else if (theme === 'honey') {
    document.getElementById('switcher-id').href = '/themes/honey.css';
  }   // when adding new themes, be sure to also add it to /includes/layout/sidebar.html and style.css under "switch" //
  localStorage.setItem('style', theme);
}