* {
 box-sizing: border-box;   
}
html {
    background-color: #222;
}
body {
    margin: 20px;
}
body, h1, h2, h3 {
    color: #fff;
}

header, main, footer {
    max-width: 1000px;
    margin: 0 auto;
}

section {
    border: 1px solid #666;
    padding: 15px;
    margin: 10px 0;
}
.height-100-75 {
    height: calc(100% - 75px);
}

#bookmark {
    background-color: #333;
}

#img {
    z-index: -1;
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    opacity: .5;
    background-image: url(../img/lorem.png);
    background-repeat: no-repeat;
    background-size: contain;
    background-position: top right;
}