header {
    display: flex;
    justify-content: space-between;
    position: fixed;
    top: 5%;
    left: -139.17px;
    background-color: gray;
    -webkit-box-shadow: 0px 0px 13px 1px rgba(0, 0, 0, 0.63);
    -moz-box-shadow: 0px 0px 13px 1px rgba(0, 0, 0, 0.63);
    box-shadow: 0px 0px 13px 1px rgba(0, 0, 0, 0.63);
}

header ul {
    list-style-type: none;
    text-align: left;
    margin: 10px;
    padding: 0;
}

header li {
    color: white;
    padding: 2px;
}

#arrow {
    width: 50px;
    transform: rotate(-90deg);
    align-self: center;
    transition: all 0.5s;
    cursor: pointer;
}

.show-menu {
    animation: showmenu 0.2s;
    animation-fill-mode: forwards;
}

@keyframes showmenu {
    0% {
        left: -139.17px;
    }
    100% {
        left: 0;
    }
}

.hide-menu {
    animation: hidemenu 0.2s;
    animation-fill-mode: forwards;
}

@keyframes hidemenu {
    0% {
        left: 0;
    }
    100% {
        left: -139.17px;
    }
}