<button class="button1">Add content to main content</button>

<div class="wrapper">
  <div class="header">HEADER</div>
  <div class="main">
    <div class="box sidebar">Left-sidebar</div>
    <div class="box content">Main Content</div>
    <div class="box sidebar">Right-sidebar</div>
  </div>
  <div class="footer">FOOTER</div>
</div>

body {
  background: @beige;
  color: black;
}

.wrapper {
  height: 100%;
  display: table;
  width: 100%;
  text-align: center;
}

.header {
  display: table-row;
  height: 1px;
  background: @yellow;
}

.main {
  height: 100%;
  display: table;
  width: 100%;
}

.box {
  display: table-cell;
}

.sidebar {
  width: 100px;
  background: @orange;
}

.footer {
  display: table-row;
  height:1px;
  background: @green;
  color: @beige;
}

/*====== Ignore section below ======*/

@orange: #BD4932;
@yellow: #FFD34E;
@green: #105B63;
@beige: #FFFAD5;


/* Basic Style*/
* { margin:0; padding:0;}
html, body { height: 100%; }
button { padding: 5px 10px;position:fixed;bottom: 50px;right:0px;left:0px;margin:auto; width: 200px; display: block; -webkit-appearance: none;background: @orange; outline: none;  border: 2px solid #DB9E36; color: @yellow; border-radius: 10px; box-shadow: 0 2px 3px rgba(0,0,0,0.5);cursor: pointer;}
button:active {border-color: @beige; color:@beige;}

document.querySelector(".button1").addEventListener("click", function(){
    var element = document.createElement("div");
    element.innerHTML = "<p>Additional Line</p><p>Additional Line</p><p>Additional Line</p>";
  document.querySelector(".content").appendChild(element);
});