Rocket.Chat/app/meteor-autocomplete/client/autocomplete.css

.-autocomplete-container {
    position: absolute;
    background: white;
    border: 1px solid #DDD;
    border-radius: 3px;
    box-shadow: 0 0 5px rgba(0, 0, 0, 0.1);
    min-width: 180px;
    z-index: 1000;
}

.-autocomplete-list {
    list-style: none;
    margin: 0;
    padding: 0;
}

.-autocomplete-item {
    display: block;
    padding: 5px 10px;
    border-bottom: 1px solid #DDD;
}

.-autocomplete-item.selected {
    color: white;
    background: #4183C4;
    text-decoration: none;
}