main.css

body {
    margin: 60px auto;
    width: 70%;
}
nav ul, footer ul {
    font-family:'Helvetica', 'Arial', 'Sans-Serif';
    padding: 0px;
    list-style: none;
    font-weight: bold;
}
nav ul li, footer ul li {
    display: inline;
    margin-right: 20px;
}
a {
    text-decoration: none;
    color: #999;
}
a:hover {
    text-decoration: underline;
}
h1 {
    font-size: 3em;
    font-family:'Helvetica', 'Arial', 'Sans-Serif';
}
p {
    font-size: 1em;
    line-height: 1.4em;
    color: #333;
}
footer {
    border-top: 1px solid #d5d5d5;
    font-size: .8em;
}

ul.posts { 
    margin: 20px auto 40px; 
    font-size: 1.5em;
}

ul.posts li {
    list-style: none;
}
.container {
  position: relative;
  display: inline-block;
  width: 100%;
  max-width: 400px;
}

.container img {
  width: 100%;
  height: auto;
}

.container .btn {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  -ms-transform: translate(-50%, -50%);
  background-color: #0000;
  color: white;
  font-size: 16px;
  padding: 100px 190px;
  border: none;
  cursor: pointer;
  border-radius: 5px;
  text-align: center;
}

.container .btn:hover {
  background-color: #0000;
}