Random background color and text (Pure JS)

HTML

<div class="monobg">
    <img src="http://chloesilver.ca/mono1.png" />
</div>
<p class="legend">Some text</p>

CSS

.blue {
    color: blue;
}
.monobg {
    background-color: red;
    padding:10px;
    width:60px;
}
.blue .monobg {
    background: blue;
}
.red {
    color: red;
}
.red .monobg {
    background: red;
}
.green {
    color: green;
}
.green .monobg {
    background: green;
}

JavaScript

function setColor() {
    var colors = [" red", " green", " blue"],
        css = colors[Math.floor(Math.random() * colors.length)];
    document.querySelector("body").className = document.querySelector("body").className + css;
}

window.addEventListener("load", setColor);