.header-horizontal-line {
    position: absolute;
    bottom: 0;
    left: 0;
    width: 100%;
    height: 1px;
    z-index: var(--z-above);
    transition: opacity var(--transition-speed) var(--transition-timing);
    background: linear-gradient(
        90deg,
        var(--primary-gradient-1),
        var(--primary-gradient-2),
        var(--primary-gradient-3),
        var(--primary-gradient-4)
    );
}