Change content width in default Learning Mode theme

How do I change the default main content width of 900px in the default Learning Mode theme to a custom number?

I wanna change it to 100% to get a full width image or color of some block groups. Then use 900 px for inner content.

Now when I’m editing the sensei sensei-course-theme (Lesson (Learning Mode – Standard)) “Layout” sizes then it’s set to my site default numbers. And even if I change those to 100%, it’s still stuck in 900px at the live site.

Is it possible to maybe copy the learning mode theme to my theme in a subfolder and then change the numbers in the theme.json file? If so, where do I store that theme?