can anyone tell me

THANK YOU SO MUCH!!!!!!!!

In the CSS window under Edit:

body {
background-color: #ff0066;
}

If you want another color you can find the hexadecimal code with https://www.w3schools.com/colors/colors_picker.asp

If you want an image as background instead do:

body {
background-image: url("https://i.ytimg.com/vi/NrdN8nAsccI/maxresdefault.jpg");
}

You can change the url to the exact url to any picture you want to use!

Good luck!