Change a window’s background colour in Preview on Mac
Preview shows a grey background behind images and PDFs. In an image or PDF, you see the background if you expand the window larger than the size of the document; in an image, you can also see the background through any transparent sections. You can change the background colour.
In the Preview app on your Mac, choose Preview > Preferences, then click General.
Click the “Window background” colour well, then choose a colour.