This would be a nice addition to make the dark theme in Paint.NET blend in better with Windows.
Essentially there is an attribute you can you to paint the titlebar black, and is what is used by File Explorer, and other applications, like DBeaver and Windows Terminal.
This is achievable by manually applying the following on the window.
DWMWA_USE_IMMERSIVE_DARK_MODE
Here is a stackoverflow post about it.
https://stackoverflow.com/a/62811758/5001796