Answers for "css when screen is smaller than"

css if screen size less than

@media (max-width:960px) { css... } //nothing with screen size bigger than 960px

@media (min-width:960px) { css... } //nothing with screen size smaller than 960px

css media screen

@media (max-width: 991px){
    .mobile{
        display: block;
    }
}