43 Folders had a post about Nocturne, a program that inverts your display colours. You can test this yourself before downloading by pressing CMD-option-ctrl-8, but Nocturne adds some additional preferences.
I've started using Nocturne to reduce my Terminal Illness. Once my eyes start to hurt I invert the screen to reduce my eye strain. I've found the white-on-black colour scheme perfect for reading mail, which I spend at least 2 hours a day doing.
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