Check out example codes for "background color". It will help you in understanding the concepts better.

Code Example 1

html,body {
  background-color: red;
}

Code Example 2

body {
  background-color: rgb(255,255,255);
}

Code Example 3

body {
  background-color: green;
}

Code Example 4

body {
  background-color: lightblue;
}

Code Example 5

div {
 opacity:25% ; 
}

Code Example 6

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

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