var x = document.querySelector(".myclass");
var y = document.getElementById("myH1");
<input type="button" value="night" onlclick="
document.querySelector('body').style.backgroundColor = "black";
document.querySelector('body').style.color = "white";
">
<input type="button" value="day" onlclick="
document.querySelector('body').style.backgroundColor = "white";
document.querySelector('body').style.color = "black";
">