.index {
    display: flex;
    flex-direction: column;
    height: 100%;
    max-height: 100vh;
    overflow: hidden;
}
@media(max-width: 900px) { .index {
    max-height: initial;
    overflow: auto;
} }

.index-menu {
    width: 100%;
    display: flex;
    justify-content: center;
    background-image: linear-gradient(0deg, #111 0%, #222 50%, #333 50%, #555 100%);
    flex: 0 0 auto;
    box-shadow: 0 0 1em rgba(0, 0, 0, .8);
}
.index-menu a {
    display: block;
    color: #fff;
    text-decoration: none;
    font-size: 1.5em;
    line-height: 1.2em;
    padding: 5px 10px;
    transition: background-color .1s;
}
.index-menu a:hover,
.index-menu a:focus {
    background-color: rgba(255, 255, 255, .1);
}
.index-menu a:active {
    background-color: rgba(127, 127, 127, .1);
}
@media (max-width: 600px) {
    .index-menu {
        flex-wrap: wrap;
    }
    .index-menu a {
        min-width: 200px;
        margin: 2px;
        padding: 10px;
        text-align: center;
    }
}