style.css

* {
    box-sizing: border-box;
}

body {
    font-family: Arial, Helvetica, sans-serif;
    font-size: 16px;
    color: #666;
}

h1, h2, a, th {
    color: #222;
}

code {
    font-family: Consolas, 'Courier New', Courier, monospace;
}

#num {
    width: 100%;
    font-size: .95em;
    padding: .7em;
    border: 1px #222 solid;
    border-radius:.5em;
}

.error {
    outline-color: #EF5350;
    border-color: #EF5350!important;
}

#num-message {
    margin-top: .7em;
    font-size: .9em;
    margin-left: .7em;
    color: #EF5350;
}

button {
    background-color: #222;
    color: white;
    border: none;
    border-radius: .5em;
    padding: .7em 1em;
    text-transform: uppercase;
}

p.warn {
    background-color: #FFEBEE;
    border-radius: .7em;
    border: 1px #EF5350 solid;
    padding: 1em;
    color: #222;
}

table {
    border-collapse: collapse;
}

th, td {
    border: 1px #ccc solid;
    padding: .7em;
}

.one {
    font-weight: bold;
    color: #222;
}

.even {
    color: green;
}

.odd {
    color: red;
}