header backdrop navigation menu - transparent to color when scrolling

Site URL: https://elizavolk-healingarts.squarespace.com

Hello! 

I am wondering if there is a way to keep my menu transparent WITH  the backdrop menu transparent too at the top of my website, and give a background color to this backdrop menu when we start toi scroll down? (fixed position header) 

when I use use a css code to make it transparent, (code on the pictures) it is great at the top, but I'd like to apply tis transparent effect only then and not when we scroll. 

It seams that it is possible with code injection but my client doesn't have the advanced version. I hope that make sense,  I am very grateful for any idea / solutions. Thank you so so much!

1st  & 2nd picture : what I have now at the top of the page and when I scroll (fixed position)

3rd picture : What I want at the top of the page keeping the second picture color when I scroll

Try adding to Design > Custom CSS

/* Nav transparent */
header#header:not(.shrink) .header-nav-folder-content {
    background-color: transparent !important;
}

That worked perfectly! Thank you for your precious help 🙏🏻