Добавление затемнения на background

shadow_bg.css

.bg {
	background-image: url('img/img.jpg');
	background-repeat: no-repeat;
	background-position: top center;
	background-size: cover;
	height: 100vh;
	width: 100%;
	position: relative;
	-webkit-transition: all 0.4s;
	-moz-transition: all 0.4s;
	transition: all 0.4s;
}
.bg:before {
	content: "";
	position:absolute;
	width: 100%;
	height: 100%;
	background: rgba(0,0,0,0.5);
}