.contents1 {
    background-color: #FFD500;
    color: #000;
    padding: 60px 30px 120px;
}
.contents2 {
    background-color: #000;
    color: #FFD500;
    padding: 60px 30px;
    position: relative;
    z-index: 0;
}
.contents3 {
    background-color: #FFD500;
    color: #000;
    padding: 120px 30px 60px;
}
.contents2::before {
    /* 上の斜め */
    content: '';
    display: block;
    /* 斜めの調整 */
    border-top: 60px solid transparent;
    border-right: 100vw solid #000;
    border-bottom: 60px solid #000;
    border-left: 100vw solid transparent;
    /* 斜めの位置の調整 */
    position: absolute;
    z-index: -1;
    left: 0;
    top: -120px;
}
.contents2::after {
    /*下の斜め */
    content: '';
    display: block;
    /* 斜めの調整 */
    border-top: 60px solid #000;
    border-right: 100vw solid transparent;
    border-bottom: 60px solid transparent;
    border-left: 100vw solid #000;
    /* 斜めの位置の調整 */
    position: absolute;
    z-index: -1;
    right: 0;
    bottom: -120px;
}