Overlay doesn't take full page - html

Question

I am working on a new page for my website, and i got a great image for the background, but i would like to use an overlay to make the image darker. But the overlay doesn't take the full page !
I have already tried a lot of things but nothing worked, and this is my html and css code :
/* //////////////////////// [ Background ] //////////////////////// */
body, html
{
height: 100%;
background-color: #fff;
-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
box-sizing: border-box;
}
.bg-img
{
background-image: url("../../images/background/bg.jpg");
display: block;
position: absolute;
width: 100%;
height: 100%;
top: 0;
left: 0;
background-position: center;
background-repeat: no-repeat;
background-size: cover;
}
.overlay
{
position: relative;
z-index: 1;
width: 100%;
min-height: 100vh;
}
.overlay::before
{
content: "";
display: block;
position: absolute;
z-index: -1;
width: 100%;
height: 100%;
top: 0;
left: 0;
background-color: rgba(0,0,0,0.8);
}
<div class="bg-img"></div>
<div class="overlay"></div>
I think you understood what i would like. That's could be very nice if someone could help me to make this overlay on the full page.

Answer 1

Add margin: 0 to the body:
body,
html {
height: 100%;
background-color: #fff;
box-sizing: border-box;
}
body {
margin: 0;
}
.bg-img {
background-image: url("https://picsum.photos/id/446/3200/3200");
display: block;
position: absolute;
width: 100%;
height: 100%;
top: 0;
left: 0;
background-position: center;
background-repeat: no-repeat;
background-size: cover;
}
.overlay {
position: relative;
z-index: 1;
width: 100%;
min-height: 100vh;
}
.overlay::before {
content: "";
display: block;
position: absolute;
z-index: -1;
width: 100%;
height: 100%;
top: 0;
left: 0;
background-color: rgba(0, 0, 0, 0.8);
}
<div class="bg-img"></div>
<div class="overlay"></div>