FreeOffice: The Dark document function is not included in SoftMaker FreeOffice.
You can use the ribbon command Dark document 
in the Quick access toolbar to switch the document pages from their usual white to a dark display.
Note: If this button is not available, click on the 
icon on the far right of the Quick access toolbar. Select Add/remove buttons and enable Dark document.
If you click on this command once, the document pages will be displayed in dark mode. If you click on this command again, the document pages will be displayed in white again.
Switching this display has no effect on the printout of the document. For more information on how to print colored pages, see Changing the page background.