Full width sticky menu and margins

Hello, would you be able to advise please?

In the sticky header menu on the site we are developing, it looks like the menu doesn’t stretch to full width across the top.

As you scroll down and the darker coloured pictures move up the screen and behind the menu, you can see a sliver of colour at the right-hand edge (by the search icon). As the maps scroll up, this becomes even more obvious.

My designer believes it is a two-fold issue and would like to know:

1. How to make the header full width
2. How to set margins to the sides for the content

Thanks in advance,
Brenda