Dark background in LibreOffice under Windows

You can modify the application background using the appearance settings (Menu Tools -> Options -> LibreOffice -> Appearance):

The "Application Background" is the gray area around the sheets, and the "Document Background" setting affects the entire sheet (margins and editable area). Together with the "Font color" setting, you can modify the appearance entirely without altering the document (page / paragraph / character) styles.

Just as a follow up, I found something you may like too.

Under Menu Tools -> Personalization you can now choose a Firefox theme to use for LibreOffice.

Maybe it is worth a look, personally I like it.

In the latest version (6.1) this setting has been moved to Option → Application Colors.

Also, for me, setting Application background had no effect. I had to set Document background.