How to make a div cover the whole screen

You can use position: absolute; or position: fixed.
Use absolute for just making it cover the whole page.
Use fixed to make it nailed in a default position. If u use fixed, even though your page is more than 100% you cannot scroll down to see any other things.

CSS

div.any {
   position: absolute; /*position: fixed;*/
   top: 0;
   left: 0;
   width: 100%;
   height: 100%;
   /*You can add anything else to this like image, background-color, etc.*/
}

HTML

<div class="any"></div>

You can probably do that by setting the position of the div that you want to make fullscreen, to absoluteand then apply the below CSS.

top:0;
left:0;
bottom:0;
right:0;
height:100%;
width:100%;

Thus, the final css would be as follows

.fullscreen{
    position:absolute;
    top:0;
    left:0;
    bottom:0;
    right:0;
    height:100%;
    width:100%;
}

You could use viewport height as your height value:

.main {
    height: 100vh;
    background-color: green;
}

<div class="main">
  CONTENT
</div>

Using height: 100vh means the element in question always be 100% height of the viewport a user / devie has.

More info: https://web-design-weekly.com/2014/11/18/viewport-units-vw-vh-vmin-vmax/