Custom Scrollbar | UI Design | HTML CSS

Custom scrollbars are getting popular, and you might have come across websites that have unique scrollbars

[thats.secret]

Hey, two questions:
How can i hide the scrollbar and make it visible only on scroll?
How can i make the "track" transparent or unvisible..i tried quite a bit but didnt work?
Look i already have you have an idea?

/* width */
::-webkit-scrollbar {
width:10px;
}
/* Track */
::-webkit-scrollbar-track {
background: green;
}
/* Handle */
::-webkit-scrollbar-thumb {
border-radius: 4px;
background: linear-gradient(red, blue);
}