:root{
    --first-color: #ffffff;
    --bg-alpha-color: rgba(0,0,0,0.5);
}

body{
    font-family: sans-serif;
}

.bg{
    background-image: url('imagen.jpg');
    background-repeat: no-repeat;
    background-position: center center;
    background-size: cover;
}

.bg-alpha-color{
    background-color: var(--bg-alpha-color);
}