Thank you saguaros
I asked a friend to control on his Imac with a wide screen and he had the gray spaces on the sides, too. I'm sorry, but I can't tell what size the screen is. The good news is , your code did his job, as you can see here (he had to empty cache to make it work):

I couldn't ask my client yet, if it works on his PC, too. I hope so and I will let you know about.
Two things though:
1 The logo + menu navigation part is much less wide compared to the rest of the page grid, which looks a little strange to me:

2 The header image is doubled as you can see better on this page:

Is there a way to solve these 2 problems?