I went to preferences - accessibility and then changed the colours of the text and background (black to white and white to black). It worked in the acrobat app because the colours really did change as I wanted. but then when both saving and exporting the pdf, it still looks the same. I want to view my pdfs in preview, not acrobat. So if this option doesn't actually change the pdf itself but just views it differently in the app, then it is useless.