<div>
  <p>背景に透過カラー</p>
</div>

body {
background: linear-gradient(rgba(0, 0, 0, 0.6), rgba(0, 0, 0, 0.6)),
  url(http://design-levelup.com/wp-content/uploads/2018/09/desk-3139127_1280.jpg);
  background-size: cover;
}
p{
  margin:100px auto;
  text-align:center;
  font-family:sans-serif;
  font-size:30px;
  font-weight:bold;
  letter-spacing:5px;
  color:#fff;
}