style.css

body {
  margin: 0;
}

.container {
  width: 85%;
  max-width: 1170px;
  margin: 0 auto;
}

.clearfix {
  clear: both;
}

/* header */

#header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  z-index: 100;
  background-color: #274358;
}

#logo {
  float: left;
}

.nav {
  float: right;
  list-style: none;
  margin: 0px;
  padding: 0px;
}

.nav li {
  display: inline-block;
}

.nav li a {
  color: #fff;
  text-decoration: none;
  padding: 10px 12px;
  display: block;
}

/* banner */

.banner-bg {
  background-image: url("http://picsum.photos/1170/600?image=1072");
  width: 100%;
  height: 450px;
  background-position: center;
  background-size: cover;
  margin-top: 45px;
}

/* content */

#content {
  margin-top: 15px;
}

.content-box {
  display: inline-block;
  width: 30%;
  margin-left: 15px;
}

.content-box img {
  width: 100%;
}