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Apple's OSX has built in aids for disability which cover this too... My son needs it but I do this too from time to time. It's system wide...

Ctrl-Opt-Cmd-8 will toggle between black on white and white on black, for all apps.

(Depending on your Apple experience level... no offence intended... Opt is probably called Alt on your keyboard and Cmd is that funny Apple/Pretzel key...)