revuo-weekly/_includes/theme-switcher.html

<!-- Unfortunately, targeting the parent is not a thing in CSS. This is a workaround. -->
<input class="nojs-toggle" id="theme-switcher" type="checkbox" name="theme-switcher">
<label class="switch" for="theme-switcher" id="main-theme-switcher">
    <span class="slider round"></span>
</label>