Full background

<div class="container">
...
</div>
.container {
/* Center the content */
align-items: center;
display: flex;
flex-direction: column;
justify-content: center;
/* Take full size */
height: 100vh;
width: 100%;
/* Background */
background: url('/assets/full-background.jpeg') center center / cover no-repeat;
}

See also

Video background