Answer 1

You are changing the div background not the body's.

Change the function to
function dark(){
document.body.style.backgroundColor="black";
document.body.style.color="white";
}

or in ES6

function dark(){
Object.assign(document.body.style, {
backgroundColor: 'black',
color: 'white'
});
}

Answer 2

Zainul Khan Give some height to your div.

y.style.height = '100vh'

or change the background color of body tag. You are changing the background color of div.