Before:

After:

I like it, FWIW.

Me too.

Perhaps the header in subnav ("Schriftarten") needs to be darker then to increase contrast.

I think it is intended that the headings have low contrast.
The user knows that "Schriftarten" is selected, as it is highlighted in the primary column.

In the second column it is rather unimportant to have it there at all. So, giving it a solid black would be too much in my opinion. And an additional shade would be one shade too many in the palette.
Any other opinions?

That begs the question: why even have headers, if they're unimportant, deliberately low-contrast, and duplicate existing status information shown elsewhere?

They are important in the primary column to structure the list items.
Yet, the list items are more important than the headings, so it is a good idea to have the headings in lower contrast, so that they create less visual attention.

The heading in the second column in not as important as the ones in the primary column, but it still is a nice little reminder.
Therefore I think that the heading in the second column should remain, just to have a little hint on what this column is about.

Giving the secondary heading a solid black color would however give it a more important role than the headings in the primary column. This is what I want to avoid.

On second thought, the secondary heading acts like a button for small screens.
This means, now that the background is the same as buttonBackground, it will look like a disabled button.

I will change the text color to solid black.

ok for the top toolbar to be darker.

the headers in the listviews should stay like they are now tugh

@mart ?