.hero {
    height: 100vh;
    background: url("../img/server_banner.png") center/cover fixed;

    /* 中央揃え修正 */
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;

    text-align: center;
    padding-top: 0;
    /* 不要 */
}

.join-btn {
    margin-top: 25px;
    padding: 14px 40px;
    background: #5865f2;
    color: #fff;
    font-size: 1.2rem;
    border-radius: 10px;
    text-decoration: none;
    transition: 0.3s;
}

.join-btn:hover {
    transform: scale(1.05);
}

body.light-mode .sub-btn {
    color: #222;
}

body.dark-mode .sub-btn {
    color: #fff;
}