Welcome to the forums! I can answer your second question easily enough:
- Select a paragraph that’s formatted exactly as you would like to have it. Do all the paintbrush work you want to do on it. Be sure it’s absolutely the default formatting you want.
- Tap the Paintbrush icon again, then tap “Formatting Options,” fourth line down.
- Tap “Set as Default Formatting”, fourth line on the screen.
Now, Default Formatting only applies automatically to NEW documents. So, select all the existing text (except any that you want to have non-default formatting!), then tap Paintbrush: Formatting Options: Use Default Formatting. Now if you insert or add new text anywhere in that document, you should get the default you just created. Any new document in the project will have your new default automatically. Unfortunately, you can only convert your existing text to your preferred default one document at a time.
As for question #1 regarding dark mode on iOS, the short answer is “no.” If you select any text colour other than “no colour”, even pure white, the text will show a less intense grey. So your choices are grey, and greyer. I’ll admit I just don’t use dark mode on iOS for this very reason.