gist:blackwhiteindex.html

<button id="turnBlack">Black</button>
<button id="turnWhite">White</button>
<script>
    var turnBlackButton = document.getElementById("turnBlack")
    var turnWhiteButton = document.getElementById("turnWhite") 
    function turnBlack() {
       document.body.style.background = 'black'
    }
    function turnWhite() {
        document.body.style.background = 'white'
    }
    turnBlackButton.onclick = turnBlack
    turnWhiteButton.onclick = turnWhite
</script>