This javascript will do the job (used it in other projects):
<script>
function init() {
window.addEventListener('scroll', function(e){
var distanceY = window.pageYOffset || document.documentElement.scrollTop,
shrinkOn = 200,
header = document.querySelector("headertje");
if (distanceY > shrinkOn) {
classie.add(header,"smaller");
} else {
if (classie.has(header,"smaller")) {
classie.remove(header,"smaller");
}
}
});
}
window.onload = init();
</script>
Will try to use this in GeneratetPress. It also uses Classie (http://callmenick.com/post/add-remove-and-check-classes)