Now many sites use this blackout, but I have not found how to do it. For example, the header has a picture on the background, and a text on top of it. But in order to read the text, a translucent black background, such as a background, is superimposed over the image. How to make it? For header there are such properties:
header { height: 538px; width: 100%; background: url(../img/header-bg.png) no-repeat center center; background-size: cover; }