[e.cash] Add navbar background change on scroll

Summary: Adding back the background change on scroll functionality now that the page has more content to scroll.

Test Plan:
@bot preview-e.cash
Scroll up and down the page and you should see navbar background change to an a darker background when you scroll down

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D13900