Black color overlay is set to 0.8 (20%) at default.
Users can change it by clicking F12, navigate elements, select HTML, there you can see:
html{
background: black;
opacity: 0.8;
}
Where you can change opacity and color if needed.
Image showing where you can change it:
https://i.ibb.co/82YB3zp/Color-Dimmer.jpg