Change the Header in Theme Solon

Hi,
I´m sorry my english is not the best. But I try my best 😉

I would like to change my header in Theme Solon. Actually it is easy, but the score is not what I wont. When I upload the Picture, the Header is not over the general of my screen. I can´t find a function to change this, even in editor.
My secondbest idea was to change the backround color behind the header. But it doesn´t work….

I hope you understand my problem and someone can help.
Sandra