friendenstein/templates/header.html

{% if config.extra.header_nav %}
<nav class="navBar">
  {% for nav_item in config.extra.header_nav %}
  {% set current_nav_item = nav_item[lang] %}
  {% if current_nav_item and current_nav_item.name %}
  <a href="{{ get_url(path=current_nav_item.url) }}" {% if current_nav_item.new_tab %}target="_blank"
    rel="noreferrer noopener" {% endif %}>{{ current_nav_item.name }}</a>
  {% endif %}
  {% endfor %}

  <div class="themeSwitch">
    {% if not config.extra.default_theme %}
    <button class="themeButton light" onclick="setTheme('light')" title="Light mode"><svg class="icons icons__background"><use href="{{ get_url(path='icons.svg#lightMode', trailing_slash=false) | safe }}"></use></svg></button>
    <button class="themeButton dark" onclick="setTheme('dark')" title="Dark mode"><svg class="icons icons__background"><use href="{{ get_url(path='icons.svg#darkMode', trailing_slash=false) | safe }}"></use></svg></button>

    {% elif config.extra.default_theme and config.extra.default_theme == "light" %}
    <button class="themeButton light" onclick="setTheme('light')" title="Light mode"><svg class="icons icons__background"> <use href="{{ get_url(path='icons.svg#lightMode', trailing_slash=false) | safe }}"></use></svg></button>
    <button class="themeButton dark" onclick="setTheme('dark')" title="Dark mode"><svg class="icons icons__background"><use href="{{ get_url(path='icons.svg#darkMode', trailing_slash=false) | safe }}"></use></svg></button>

    {% elif config.extra.default_theme and config.extra.default_theme == "dark" %}
    <button class="themeButton dark" onclick="setTheme('dark')" title="Dark mode"><svg class="icons icons__background"><use href="{{ get_url(path='icons.svg#darkMode', trailing_slash=false) | safe }}"></use></svg></button>
    <button class="themeButton light" onclick="setTheme('light')" title="Light mode"><svg class="icons icons__background"><use href="{{ get_url(path='icons.svg#lightMode', trailing_slash=false) | safe }}"></use></svg></button>
    {% endif %}
  </div>
</nav>
{% endif %}

<script>
  const setTheme = (theme) => {
    document.documentElement.className = theme;
    localStorage.setItem('theme', theme);
  }

  const hasCodeRun = localStorage.getItem('hasCodeRun');

  if (!hasCodeRun) {
    const defaultTheme = "{{ config.extra.default_theme }}";
    setTheme(defaultTheme);
    localStorage.setItem('hasCodeRun', 'true');
  }

  const getTheme = () => {
    const theme = localStorage.getItem('theme');
    if (theme) {
      setTheme(theme);
    }
  }

  getTheme();
</script>