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

Code Example 1

body {
  background-position: center;
  background-repeat: no-repeat;
  background-size: cover;
}

Code Example 2

body { 
  background: url(img/bg-image.jpg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

Code Example 3

#mainBackground{
    width: 100%;
    height: 100%;
    max-height: 100%;
    margin: 0;
    padding: 0;
    background-image: url("https://images.unsplash.com/photo-1487058792275-0ad4aaf24ca7?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=750&q=80");
    background-size:100% 100%;
    background-repeat: no-repeat;

}
html, body {
    height: 100%;
    margin: 0;
    padding: 0;
}

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