I'm trying to make a fixed header in a fluid layout that is transparent at the start but when y scroll down the background of the header turns black.
If also there is a way to make an effect it would be even better.
Please check this example Creative 1
Scroll motion effects are possible only with fixed width layouts.
In fluid width layouts you would need a 3rd party widget like scroll reveal from muse-themes.com (it´s free).
As far as I´m experienced in a fluid width layout it is not possible without 3rd party widgets.
If watching closely enough you´ll see that your example is done with fixed width breakpoints as well.
Watch here for tutorials : scroll motion menu in muse - YouTube
You´re welcome all time