Antimatter Scrolled transition

Hi,

I'm trying to get rid of the transition from not scrolled to scrolled on the navigation.

I'm using Antimatter theme and whilst scrolled, there is a small window where it doesn't have a white background and looks strange:

Before and after this point it works fine. Once scrolled it looks how i want it, before scrolled it looks how i want it, but during the first part of the scroll it looks like the above.

I'm using this to stop the modular showcase from stretching to the top:

.fullwidth #body {
padding-top: 5rem;
}

Thanks in advance.