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Change a window’s background colour in Preview on Mac
Preview shows a grey background behind PDFs and images. In a PDF or image, 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 > Settings, then click General.
Click the “Window background” colour well, then choose a colour.