The easiest way would probably be to click on the page background color (button under the live view) and set the page background to black.  That way all unused space on the page will appear black.

Regards,
Mike

The font size of the popup hints is one of the improvements in the soon-to-be-released 2024 edition.  There isn't any dropper selection for page background color but you could use a workaround: click on one of the border colors on the Prints tab and use "Pick with dropper" and click on the color you want in the thumbnail.  Then click on that same border color again and select "Choose Color" and note the RGB values from the color you just picked on the right side of the dialog.  Then you can just use those same values in the page background color.

Regards,
Mike