Header and Menu Opacity

Would like to set the Header, Footer and Menu backgrounds to 50 or 60% opacity but cannot seem to get it to work. These areas are white or dark grey in my design, with a full screen image slider behind. I’d like the images to be visible through the header/footer/menu containers.

I’ve been trying to add the appropriate opacity tags to .site-container but it seems the property is coming from elsewhere.

Anyone know where I need to add these properties?

Thanks.