.header {
    height: 50px;
    background-color: salmon;
    transition: all 2s ease-in-out;
}
.header.about {
    background-color: deeppink;
    height: 150px;
}

.animation {
    height: 10px;
    background-color: palegreen;
    position: fixed;
    bottom: 0;
    left: 0;
    right: 0;
}

.animation.in {
    animation: animateIn 0.6s cubic-bezier(0.8, 0, 0.9, 1);
    height: 100%;
    background-color: darkslategray;
}

.animation.out {
    animation: animateOut 0.6s cubic-bezier(0.1, 0, 0.2, 1);
    height: 0;
    background-color: darkslategray;
}

@keyframes animateIn {
    0% {
        height: 0;
        background-color: palegreen;
    }
    100% {
        height: 100%;
        background-color: darkslategrey;
    }
}

@keyframes animateOut {
    0% {
        height: 100%;
        top: 0;
        background-color: darkslategrey;
    }
    100% {
        height: 0;
        top: 0;
        background-color: palegreen;
    }
}