@3dickulus: This regulare page is supposed to show full width.
I created a template-full-width.php template in the child theme and appended the necessary code according to these instructions:
but it still doesn’t spread across the WHOLE browser width (or preferrably 90%)
The content area still seems to be limited. The following picture should show with a max width of up to 2560*1440, but it’s shrinking to the content-container(or whatever that area is called. We need to extend that area)