Logo + Menu header like on Demo version

Hi guys I'd like to know how to edit the behaviour of the logo + menu header. On my site (which isn't live yet, sorry I can't share the link), my logo+menu header has a solid background. I'd like it to be transparent when at the top of the page, but then becoming smaller, sticky and with a slightly transparent black background. Exactly like on the demo version. I can't seem to find where to change those setting. Thank You