Footer overlapping content

[KR1275]

I want to have my footer at the bottom of every page. I see that my footer is overlapping the content. It's not possible to scroll to see the rest of the page (even worse on small screens, like mobile phones).

How can I get the footer always under the page content (and when the content is short, at the bottom of the screen).

I use custom css:

.astroid-sticky-footer {
position: absolute;
bottom: 0;
width: 100%;
}

I think the content needs 100% height, but I don't know where to enter this css. Someone who knows this?