CSS

background color css

html,body {
  background-color: red;
}

background color

//javascript type css:
document.body.style.backgroundColor = "blue";
// or inline css:
<div style="background-color: blue;"></div>

background color

document.getElementById('id').style.backgroundColor = 'green'; 

background color css

body {
  background-color: black;
}

background color

background-color: /*your color*/ ;