.card-columns .card should have margin-bottom regardless of screen size

Created by: hellow554

Currently the margin-bottom only applies to sm and above screens, which is not ideal in my opinion. I think it should apply to all screen sizes, because it gives you a little bit of spacing between the cards, which is nice. Because I have no Idea of sass, and only little of css, this is what is currently in my css file

.card-columns .card {
    margin-bottom: .75rem;
}

which has been stolen from the css file. It should be trivial to implement this if you want it.