Banner image not using max. width of screen

Hi everybody,
I’m trying to create my first wordpress site (offline at this moment) with the blossom travel theme.

My problem is that when i open the site on a big screen (2560x…) the banner image is not using the full width of the screen. (same in the live demo of the theme).
The image stops at about 3/4 of the screen width and the grey background color start and fills the rest of the screen.
Youre other themes, like Blossom Fashion is using the full width.

Do you know how to solve that. So the banner/header is always full screen (width-wise)

Thanks and regards.