I finally decided to work with a new theme, or rather, to change the existing theme. It was the classic Kubrick with a custom background image, now it's "flexible Kubrick" - basically still Kubrick in terms of layout, but I've removed the rounded corners and shadows and switched the entire layout to em as a measure - no more fixed pixel widths, but instead font-relative specifications. Let's see, maybe I'll make the theme available later.
As a result, the layout should now adapt to the user's basic font settings and generally work more gracefully with corresponding changes. However, it is still not a liquid layout, but static - just with a flexible base size.
By the way, I solved the problem with the header graphic in a very classic way: the excavator is simply cut out and placed in front of a transparent background and saved as a GIF. Combined with the right background color, the result is quite usable in my opinion. And when the size of the page changes (e.g. by increasing the base font), the image grows in width with it.
If you notice anything strange, please let me know. I know it's not a layout-technical revolution. But I'm not capable of that anyway. I just wanted to try out what an em-based layout can look like and needed something to try it out on.