.fire-background {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: -1;
    background: 
        radial-gradient(ellipse at top, #000000 0%, transparent 70%),
        linear-gradient(to bottom, 
            #000000 0%,
            #250000 30%,
            #400000 60%,
            #600000 100%
        );
    background-blend-mode: overlay;
}