I second this. I unterstand that you may think the blue header does represent the corporate identity but the shade of blue is quite heavy and does not blend well with the dark mode. If you're dying to keep it, make at least a colorless header an option for dark mode.
I´ve installed V6 alongside V5, V6 has its own folder and both versions work flawlessly together. Just take the backup file from V5 and use it with V6, it works fine. I haven't tested iCloud sync yet because there is no new beta for iOS available.