Свойство CSS border-bottom не действует

#slideshow {
   border-bottom-style: 5px solid lightgray;
}

<div id = "slideshow" class = "container-fluid">

       <!-- Full-width images with number text -->
        <div class = "mySlides">
            <div class = "numbertext">1 / 6</div>
            <img src = "y" style = "width:100%">
        </div>

       <div class = "mySlides">
           <div class = "numbertext">2 / 6</div>
           <img src = "" style = "width:100%">
       </div>

       <div class = "mySlides">
           <div class = "numbertext">3 / 6</div>
           <img src = "" style = "width:100%">
       </div>

       <!-- Next and previous buttons -->
       <a class = "prev" onclick = "plusSlides(-1)">&#10094;</a>
       <a class = "next" onclick = "plusSlides(1)">&#10095;</a>

       <!-- Image text -->
      <div class = "caption-container">
       <p id = "caption"></p>
      </div>

      <!-- Thumbnail images -->
      <div class = "row">
           <div class = "column">
                 <img class = "demo" src = "y" style = "width:100%" 
            onclick = "currentSlide(1)" alt = "The Girl in Room 105">
           </div>
          <div class = "column"> 
             <img class = "demo cursor" src = "" style = "width:100%" 
              onclick = "currentSlide(2)" alt = "">
          </div>
          <div class = "column">
              <img class = "demo cursor" src = ""style = "width:100%" 
              onclick = "currentSlide(3)" alt = "">
          </div>

    </div>