Looking for custom code to make transparent hear go to white when fixed

I have a transparent header. I added the code:

.Header {
position: fixed !important; 
z-index: 1000;
width: 100%;
}

to make it fixed. I'd like only the fixed header to have a white background. Is there code I can add to achieve this? I added background: white, but it changed my transparent header to white - only want the sticky header white.

What is your site url? We can check easier

If SS 7.1, you can enable fixed header by use this option

then use this CSS code to change sticky header background color

header#header.shrink {
	background-color: white !important;
}