First of all thank you for the hints. I now played around a little bit with the "custom header based on File". As I am not a professional PHP programmer with deep knowledge of HTML and CSS I have a problem now.
When I define a header height of 200 pixels (as an example), I would like to have the logo and the menu in two different "rows" within these 200 pixels. I just added an additional row to the header.php file but then I have an additional row that is also 200 pixels in height.
The solution I would like to have in "sp-header" is a height of 200 pixels containing two rows (one for the logo with a height of 150 px and one for the menu with a height of 50 px).
I hope you understand what I would like to define.
Frank