css make background image fill entire page code example
Example: set background image full screen css
#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;
}