body {
    font-family: one, "Microsoft JhengHei UI";
    text-align: center;
    overflow: hidden auto;
    margin: 0;
    padding-top: 40px;
    color: #232425;
}
body > background {
    position: fixed;
    z-index: -1;
    left: 0; top: 0; width: 100vw; height: 100vh;
    background-image: url(background.png);
    background-size: cover;
    background-position: center;
    filter: blur(15px);
    transform: scale(1.2);
}

::selection {
    background-color: #C0AD7E;
    color: #FFF;
}

#book {
    display: flex;
    position: absolute;
    left: 50%; top: 50%; transform: translate(-50%, -50%);
    width: 800px;
    height: 560px;
    background-color: #E1E1E1;
    border-radius: 20px;
    box-shadow: 0 0 0 6px #000, 
        0 0 0 8px #333;
    padding: 20px 30px 60px 30px;
}
#book > nav {
    width: 200px;
    font-weight: bold;
    font-size: 18px;
}
#book > nav > sr-nav-item {
    margin: 10px 0;
}
#book > nav > sr-nav-item > info {
    display: block;
    font-weight: normal;
    letter-spacing: 1px;
    font-size: 12px;
    margin-top: 5px;
    opacity: 0.5;
}
#book > main {
    flex-grow: 1;
    margin-left: 20px;
    overflow: hidden auto;
    font-weight: bold;
}
@keyframes book-main-fade {
    0% {
        transform: translate(20px);
        opacity: 0;
    }
}
#book > main > div {
    animation: book-main-fade 0.3s;
    background-color: #232425;
    color: #FFF;
    min-height: calc(100% - 20px);
    padding-bottom: 20px;
}
#book > main > div > h3 {
    background-color: #F1F1F1;
    color: #232425;
    margin: 0 0 20px 0;
    font-size: 15px;
    line-height: 30px;
    letter-spacing: 1px;
}
#book > main > div > code {
    white-space: pre-wrap;
    display: block;
    text-align: left;
    margin: 20px 20px;
    background-color: #1A1B26;
    font-size: 15px;
    font-family: "Cascadia Mono", "Microsoft JhengHei UI", monospace;
    padding: 10px;
    border-radius: 5px;
    font-weight: normal;
}
#book > tools {
    position: absolute;
    display: flex;
    bottom: 0;
    left: 0;
    width: 100%;
    height: 40px;
    border-top: 2px dotted #aaa;
}

flex-box {
    display: flex;
    justify-content: center;
}
flex-box > * {
    margin: 10px;
}