CSS Background color is not full, it does not cover the whole page properly

Make use of viewheight and viewwidth:

html, 
body, 
#background { 
  width: 100vw;
  min-height: 100vh;
  background-color: #9B59B6;
}

Perhaps make it static (without the min- prefix) and add zero padding/margin to fix a white gap.

body{background-color:#9B59B6;}

This should work. But what is the html element that #background id is set to?

I guess that is where the problem is.