<body>
  <div class="container">
    
    <h1>Titre du site</h1>
    <p>Paragraphe quelconque</p>

    <button class="btn" id="darkTrigger">Activer le thème sombre</button>
    
  </div>
  
</body>
/** Dark CSS */

.dark {
  background-color: #112F41;
  color: #ffffff;
}

.dark button {
  background: #144098;
  color: #fff;
}
$("#darkTrigger").click(function(){
  if ($("body").hasClass("dark")){
    $("body").removeClass("dark");
  }
  else{
    $("body").addClass("dark");
  }
});

$(document).ready(function () {
  var d = new Date();
  var n = d.getHours();

  if(n > 17 || n < 8){
    $("body").addClass("dark");
  }
});

External CSS

  1. https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.3/css/bootstrap.min.css

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js