Header background image just stopped working

Your tag is using 2 identifiers instead of one id="masthead masthead_custom"

class="masthead_custom" id="masthead" will correct your problem.

#masthead.masthead_custom { 
    background-image: url("./bg.jpg");
    background-color: #444;
    height: 100vh;
    background-position: center;
    background-repeat: no-repeat;
    background-size: cover;
    opacity: 0.6;
    position: relative;;
    z-index: 999999;
}

Tell me if it works.