html,
body {
    background-color: #20007c;
    background-image: url('images/clouds.jpg');
    background-size: cover;
    background-position: center;
}

.slide-right {
    animation: slide-alternate 5s ease-in-out infinite both;
}

@keyframes slide-alternate {
    0% {
        left: 50px;
    }
    50% {
        left: calc(100% - 102px);
    }
    100% {
        left: 50px;
    }
}

.circle {
    background-color: white;
    border-radius: 100px;
    border: 1px solid black;
    box-shadow: 10px 5px 5px #000;
    color: black;
    font-family: Arial, sans-serif;
    font-size: 2em;
    font-weight: bold;
    height: 100px;
    left: calc(50% - 51px);
    line-height: 100px;
    position: absolute;
    text-align: center;
    top: calc(50% - 51px);
    transition: opacity 1s;
    width: 100px;
}

#start {
    cursor: pointer;
    opacity: 1;
    z-index: 1;
}

#follow {
    color: black;
    cursor: pointer;
    opacity: 0;
    z-index: 0;
}

#stop {
    cursor: pointer;
    opacity: 0;
    bottom: 10px;
    top: auto;
}

#audio {
    display: none;
}