Custom login screen's background image not covering

I am facing an issue on the custom login screen. I set the Background size to cover, but it is not covering the page.
During the chat we've checked the Custom CSS and noticed that commenting/removing a line seems to fix the issue. This is how it should be:

html body {
display: flex;
align-items: stretch;
align-content: flex-start;
background-position: 100% 100%;
/* background-size: 60%; */
}

So the "background-size: 60%;" property needs to be disabled.