gist:OnOffIndex.html

<h1 id="turnBlack">Off</h1>
<h1 id="turnWhite">On</h1>
<script>
    var turnBlackButton = document.getElementById("turnBlack")
    var turnWhiteButton = document.getElementById("turnWhite") 
    function turnBlack() {
       document.body.style.background = 'black'
       turnBlackButton.style.color = 'white'
       turnWhiteButton.style.color = 'white'
    }
    function turnWhite() {
        document.body.style.background = 'white'
        turnBlackButton.style.color = 'black'
        turnWhiteButton.style.color = 'black'
    }
    turnBlackButton.onclick = turnBlack
    turnWhiteButton.onclick = turnWhite
</script>