Set Pages to Dark Mode

Pages does not feature a dark mode setting. What you see is what you get when the System appearance setting is in Dark Mode.

You might be tempted to use that black document template with its default white text but that black content will export as black or gray with white text depending on the export choice. That would not endear you to any recipient of that document.

Microsoft Word has Preference settings (e.g. General > Personalize) that allow for a black background with white text that responds to macOS Dark mode and that does not save documents with a black background. So does the free LibreOffice Writer.