[Theme: Gateway] header image issue

Hi team,
I’ve installed the theme Gateway on my website : http://kinesiologie-reiki.ch/
I don’t understand why on the demo site here : https://gatewaydemo.wordpress.com/
the header background is very big and fixed while mine is thin and scroll.
I can’t find the option to get the header background fixed in the customizer window.
I’d like exactly the same behavior and visual as the demo website.
I wonder if you can help me

Best Regards
Joel