script.js

var css = document.querySelector("h3");
var color1 = document.querySelector(".color1");
var color2 = document.querySelector(".color2");
var body = document.getElementById('gradient');
var btn = document.querySelector("button");

body.style.background = "linear-gradient(to right, " 
	+ color1.value
	+", "
	+ color2.value
	+")";
css.textContent = body.style.background + ";";

function setRandomColor(){
	color1.value = getRandomColor();
	color2.value = getRandomColor();
	setGradient();
}

function getRandomColor() {
  var letters = '0123456789ABCDEF';
  var color = '#';
  for (var i = 0; i < 6; i++) {
    color += letters[Math.floor(Math.random() * 16)];
  }
  return color;
}

function setGradient(){
	body.style.background =
	"linear-gradient(to right, " 
	+ color1.value
	+", "
	+ color2.value
	+")";
	css.textContent = body.style.background + ";";
}
color1.addEventListener("input", setGradient);
color2.addEventListener("input", setGradient);
btn.addEventListener("click", setRandomColor);