body {
    background: #141e30;
}

header {
    background: rgba(21, 76, 119, .3);
}

/* 第一页 */
.singlePageIntro {
    cursor: pointer;
    position: relative;
    top: -56px;
}

.singlePageIntro>img {
    position: absolute;
    right: 40px;
    top: 300px;
    width: 56px;
    visibility: hidden;
    transition: all .36s ease-in-out;
}

.singlePageIntro>img:last-child {
    position: absolute;
    bottom: 0;
    visibility: hidden;
    transition: all .36s ease-in-out;
}

iframe {
    box-sizing: border-box;
    background-color: black;
    width: 100%;
    height: 100vh;
    z-index: -1;
}

#biggestTitle {
    position: absolute;
    top: 0;
    width: 100%;
    height: 100vh;
    color: white;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    backdrop-filter: saturate(180%) blur(2px);
}

#biggestTitle>div {
    position: absolute;
    top: 0;
    width: 100%;
    height: 100vh;
    background-size: cover;
    mask-image: url(../image/beidou.png);
    -webkit-mask-image: url(../image/beidou.png);
    mask-size: cover;
    -webkit-mask-size: cover;
}

#biggestTitle img {
    width: 100%;
    height: 100%;
    object-fit: cover;
}

#iframeMask {
    width: 100vw;
    height: 100vh;
    background-color: black;
    position: absolute;
    top: 0;
}

#biggestTitle p {
    position: absolute;
    bottom: 5rem;
    letter-spacing: 1rem;
}

#biggestTitle p:last-child {
    visibility: hidden;
}

.singlePageIntro {
    color: white;
}

#lessonPage {
    position: relative;
    background: #141E30;
    display: flex;
    flex-direction: column;
    justify-content: center;
}

#lessonPage h1 {
    letter-spacing: 2rem;
    margin: 4rem auto;
    margin-bottom: 2rem;
    font-size: 3rem;
}

#lessonPage>p {
    margin: 0 auto;
    margin-bottom: 1rem;
    font-size: 1rem;
    color: #c4cbcf;
}

#lessonIntroWrapper {
    width: 75%;
    margin: 0 auto;
    margin-top: 4rem;
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
}


#lessonIntroWrapper li {
    width: 30%;
    min-width: 180px;
    list-style: none;
    margin-bottom: 5rem;
    background: rgb(172, 171, 171);
    color: black;
    display: flex;
    flex-direction: column;
    position: relative;
    border-radius: 1rem;
    box-shadow: 5px 5px 5px 1px #c6d9ed9f, -5px -5px 5px 1px rgba(255, 255, 255, 0.459);
    transition: all .36s ease-in-out;
    border: white 1px solid;
}

#lessonIntroWrapper li:hover {
    transition: all .36s ease-in-out;
    transform: scale(.975);
    background: rgb(197, 197, 197);
    box-shadow: 0 0 0 #fff, 0 0 0 #fff, 5px 3px 6px 1px #c6d9ed inset, -5px -3px 6px 1px #fff inset;
    transition: all .36s ease-in-out;
}

#lessonIntroWrapper li:active {
    background-color: rgb(199, 199, 199);
    transform: scale(.95);
    box-shadow: 1px 1px 2px 2px rgba(0, 0, 0, .4)
}

#lessonIntroWrapper img {
    width: calc(100%-2rem-2px);
    height: auto;
    border-radius: .5rem;
    margin: 1rem;
    border: white 1px solid;
    box-shadow: 0 0 14px white;
}

#lessonIntroWrapper h3 {
    padding: 0 1rem;
    margin-bottom: 0;
    font-weight: bold;
    text-align: justify;
}

#lessonIntroWrapper p {
    min-height: 5rem;
    padding: 0 1rem;
    text-align: justify;
    text-indent: 2rem;
}

#lessonIntroWrapper .videoplayedtimes {
    min-height: 1rem;
    color: grey;
    margin-top: .5rem;
    margin-bottom: 0;
    text-indent: 0rem;
}

#lessonIntroWrapper a {
    text-align: right;
    text-decoration: none;
    margin: 0 1rem;
    margin-bottom: 1rem;
}

#lessonIntroWrapper a:hover {
    color: rgb(131, 22, 255);
}

#lessonIntroWrapper a:active {
    color: rgb(107, 43, 226);
}