example.css

@import url(http://fonts.googleapis.com/css?family=Roboto);

* {
  box-sizing: border-box;
}
body {
  font-family: 'Roboto', Helvetica, sans-serif;
  color: #212121;
  background-color: #e3f2fd;
}

button.open-modal {
  position: fixed;
  top: 20%;
  left: 50%;
  padding: 1em;
  font-size: 2em;
  width: 10em;
  margin-left: -5em;
  cursor: pointer;
  background-color: #fafafa;
  color: #212121;
  border: none;
  -webkit-box-shadow: 0 2px 5px 0 rgba(0, 0, 0, 0.16), 0 2px 10px 0 rgba(0, 0, 0, 0.12);
  -moz-box-shadow: 0 2px 5px 0 rgba(0, 0, 0, 0.16), 0 2px 10px 0 rgba(0, 0, 0, 0.12);
  box-shadow: 0 2px 5px 0 rgba(0, 0, 0, 0.16), 0 2px 10px 0 rgba(0, 0, 0, 0.12);
  border-radius: 3px;
}

.modal {
  background-color: #fafafa;
  width: 20rem;
  padding: 1rem;
  border-radius: 3px;
}
.modal-overlay {
  background-color: rgba(0, 0, 0, .3);
  display: flex;
  justify-content: center;
  align-items: center;
}