html {
   font-family: 'M PLUS 1', sans-serif;
    overflow-x: hidden;
}

main, footer {
    text-align: center;
    padding: 100px;
}

#header {
    background-color: darkgreen;
    width: 90%;
    max-width: 1000px;
    height: 70px;
    margin: 0 auto;
    display: flex;
    justify-content: space-between;
    align-items: center;
}
#header.active {
    background-color: lightblue;
}

/*非アクティブ*/
#hmb {
    width: 32px;
    height: 28px;
    cursor: pointer;
    position: relative;
    z-index: 30;
}
#hmb span {
    width: 32px;
    height: 3px;
    display: block;
    background-color: #000000;
    border-radius: 2px;
    position: absolute;
    transition: .2s;
}
#hmb span:first-child {top: 0;}
#hmb span:nth-child(2) {top: 12px;}
#hmb span:last-child {top: 24px;}
nav {
    position: fixed;
    top: 0;
    right: 0;
    width: 80%;
    max-width: 400px;
    height: 100vh;
    display: flex;
    align-items: center;
    padding: 20px;
    background-color: mediumpurple;
    z-index: 20;
    opacity: 0;
    visibility: hidden;
    transform: translateX(100%);
    transition-property: all;
    transition-duration: .2s;
}
nav ul li a {
    background-color: orange;
    display: block;
    margin-bottom: 16px;
}

/*アクティブ*/
.active #hmb span:first-child{
    top: 12px;
    transform: rotate(45deg);
}
.active #hmb span:nth-child(2){
    opacity: 0;
}
.active #hmb span:last-child{
    top: 12px;
    transform: rotate(-45deg);
}
.active nav {
    visibility: visible;
    opacity: 1;
    transform: translateX(0);
}
.active::before {
    content: '';
    position: fixed;
    top: 0;
    right: 0;
    background-color: #000000;
    width: 100vw;
    height: 100vh;
    opacity: 0.4;
    z-index: 10;
}