"sticky" section on scroll?

URL: https://www.jconstantino.net/hear2

Hi all. I am trying to add some custom styling/functionality to the first line of text on the page above ("for a list of tracks currently..."). I would like to add a background color block to that text that spans the width of the content (the distance from the logo on the top left to the end of the nav on the top right), but I would also like this new section to replace the nav bar when someone scrolls down, so that once the nav bar disappears, the new section with the link sticks to the top of the screen. I hope this makes sense, and if so, can someone assist with this? Thank you very much!

Thank you so much!