Answers for "how to keep a background for whole html."

css background image

background-image: url("image.png");
background-position: center;
background-repeat: no-repeat;
background-size: cover;

bootstrap create full screen background image

.wrapper{background: url('/assets/64531/green_suburb.jpg') no-repeat   center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;}