Fix background transition on main navbar when scrolling back to top

[cfleschhut]

Before:

Navbar background color changes from gray back to transparent abruptly

After:

Background color changes from gray smoothly back to transparent

---

Happy Hacktoberfest! 🎃

[mei-li]

In my BE eye. I see no difference at all.

@discombobulateme can you please check this?