html, body { margin: 0; padding: 0; }
header h1 { text-align: center; }
header { border-bottom: 1px solid #777; }

#main {
    padding: 20px 0;
    background: #fff;
    -webkit-transition: all 0.2s ease-in-out;
    -moz-transition: all 0.2s ease-in-out;
    -o-transition: all 0.2s ease-in-out;
    transition: all 0.2s ease-in-out;
}

#main.green {
    background: #afa;
}

#main.red {
    background: #faa;
}

#display {
    font-size: 180px;
    text-align: center;
}

#bpm {
    background: #fff;
    -webkit-transition: all 0.2s ease-in-out;
    -moz-transition: all 0.2s ease-in-out;
    -o-transition: all 0.2s ease-in-out;
    transition: all 0.2s ease-in-out;
}

#bpm.green {
    background: #afa;
}