example.css

html {
  box-sizing: border-box;
}

*, *:before, *:after {
  box-sizing: inherit;
}

body {
  font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
  font-size: 16px;
  line-height: 1.5;
  margin: 0;
  padding: 0;
}

.header {
    background: linear-gradient( rgba(0, 0, 0, 0.5), rgba(0, 0, 0, 0.5) ), url('https://source.unsplash.com/mNWsZDYUCFs/1500x1000');
    background-size: cover;
}


.hero {
  display: flex;
  align-items: center;
  justify-content: center;
  height: 400px;
  color: #ffffff;
}

.button {
  padding: 10px 16px;
  border-radius: 6px;
  background-color: transparent;
  border: 1px solid #cccccc;
}

.thumbnail {
  padding: 4px;
  line-height: 1.42857143;
  border: 1px solid #ddd;
  border-radius: 4px;
}

.text-center {
  text-align: center;
}

.image {
  display: block;
  max-width: 100%;
  height: auto;
}

.image--center {
  margin-left: auto;
  margin-right: auto;
}

.image--circle {
  border-radius: 50%;
}

.footer {
  height: 50px;
  background-color: #404040;
  margin-top: 30px;
}