Set the background image of a body of a html page to 50%, using pseudo-element ::before and the opacity: 0.5; property

customStyles.css

body::before {
	content: "";
	position: absolute;
	top: 0;
	left: 0;
	right: 0;
	bottom: 0;
	background-image: url('place-your-image-url-here.jpg');
	background-size: cover;
	background-position: center;
	opacity: 0.5;
	z-index: -1;
}

body * {
	position: relative;
	z-index: 1;
}