Header colour changing when scrolling

Hi there!

I was wondering if there was a way to change the colour of the menu header when scrolling down? I found some useful topics in this forum and managed to change the header on the Main page with this in Appearance > Customize > Additional CSS:

#header {
background-color: #5f4b8b;
}
nav #desktop-menu li a {
color: white;
}

However, when you start scrolling down the page, the background colour of the header menu changes to white. Is there a way to keep it fixed to the same colour (#5f4b8b)?

Thanks for your help!

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