Is there a way I can change the scrolling header background to be different to the set header background?

Hi, I have just started a new site and would like the header background to change colour when the user scrolls. Currently the header area has a turquoise background colour and I can see that I can change the logo in BB to another one but I would like to change the background to white so that the menu is visible:

Here is the site in development

How do I change the CSS?

I’ve tried this solution:

.fl-theme-builder-header-scrolled { background: #fff; }

but nothing happens