Hello wfed,
Welcome to Apple Support Communities! I understand it's hard to see your notes in Dark Mode because the text boxes and characters are white. I'm happy to help. You can change the background in your Notes app in order to make it darker and easier to see your words. To use a dark background for notes while Dark Mode is turned on, open Notes and choose Notes > Preferences, then select ”Use dark backgrounds for note content.”
You can read more information on Dark Mode here: How to use Dark Mode on your Mac
I hope this answers your question. Thanks for using the Apple Support Communities, and have a great day!