Answers for "change scroll bar colours"

change scrollbar color

.scrollable-element {
  scrollbar-color: red yellow; /* red is for the thumb and yellow is for the track */
}

on scroll change navbar color

$(document).ready(function(){
    $(window).scroll(function(){
        if($(window).scrollTop() > $(window).height()){
            $(".menu").css({"background-color":"transparent"});   
        }
        else{
            $(".menu").css({"background-color":"white"});
        }

    })
})