style.css

html, body {
  height: 100%;
  margin: 0;
  padding: 0;
}

header {
  position: fixed;
  width: 100%;
  height: 50px;
  background: red;
  z-index: 99999;
}

/* 
 * Header height: 50px;
 * Footer height: 50px;
 * Sidebar width: 200px;
 */

.sidebar {
  width: 200px;
  position: fixed;
  left: 0;
  top: 0;
  background: yellow;
  height: 100%;
  padding-top: 50px;
  z-index: 999;
}

.wrapper {
  box-sizing: border-box;
  padding-top: 50px;
  padding-left: 200px;
  position: relative;
  width: 100%;
  min-height: 100%;
}

.content {
  padding-bottom: 50px;
}

footer {
  bottom: 0;
  position: absolute;
  height: 50px;
  width: 100%;
  background: green;
  box-sizing: border-box;
  margin-left: 200px;
  padding-left: 200px;
  left: -200px;
}


@media (max-width: 1200px) {
  .sidebar {
    display: none;
  }

  .wrapper {
    padding-left: 0;
  }

  footer {
    padding-left: 0;
  }
}