Header Picture not centered

Hi,
I’m new to WordPress and started building a web page with the “Modern” theme. It works quite well for me, except with one issue.
I’m using a display with 3840×1080 pixel resolution. The picture, I’ve uploaded has a resolution of 3547×1995 pixel. If a display my site in a browser, using the hole screen, the following happens:

1. Menus, content and blogs are shown centered, which is expected.
2. The picture is shown with a left alignment, leaving the right border black. This also leads to the fact, that the picture is not underlaying the complete web content (see 1). This is not an expected behaviour.

I would have expected, that the header picture also gets shown centered and not with a left alignment. Is it possible, that this is a small bug in the template? I know, that such screen sizes are not usual, but I would expect them to show up more in the future.
I tried to work around this issue by overwriting the theme with own CSS code, but with less effort so far.

Can you please help me?

Christian

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