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