how to make a modal stay center of screen

//add a  onclick function to your modal button
<button type="button" class="header__button" data-toggle="modal" data-target="#signUpModal" onclick="modalCenter()">
	signup
</button>
//add below function to your scripts
function modalCenter() {
        $('#signUpModal').css({
            'display': 'flex',
            'justify-content': 'center',
            'align-items': 'center'
        })
    }