Thanks so much @tuanphan! The header opacity issue seems to be fixed. I noticed a few things after plugging in the code:
If you view this page: https://platinum-bird-789c.squarespace.com/video-multimedia
Is there a way to move the underline showing the active section ("Services" in the attached screen shot) closer to the text?
On mobile, I also noticed that the header shortened, compared to its height in the attached "before" screen shots. Before this new code, it was one height at the top of the page, then shortened a tiny bit after scrolling down. Is there a way to increase that height for mobile?
Thanks again for the help!!