Since Pages doesn’t allow document background to be changed there is little hack. Just make a text box with black background and white text. Remove the margins, and expand the text box to fill the screen.
Now you go to full screen you have a distraction free writing environment.
If you don’t want to go completely black you can change it to what ever color you want.
By using black you can save quite a bit of battery power. By using black on white it looks good even in the lowest brightness which in turn preserves the battery.
You can even set this as template and use it anytime you want.
When apple starts the iCloud service you can even have your important file synced by using Pages. So there is a big benefit in using pages for all your writing needs.