The Screen Shot is of Google Keep & I'm using Substratum Themes on my device.
Even if you enable Force Dark Mode in Developer Options - Samsung Notes still has a white background.
Samsung Notes when I open the app is black - But on the pages where you enter text it's white background.
Check if the Samsung notes app is updated to the latest version. I suggest you to try to enable "Force Dark Mode" in Developer options as suggested by Shaggyskunk1 and check if that works.
Please follow below steps for Developer option.
Settings app >> About phone >> Software information >> Tap "Build number" seven times.