Edited to add: I don't want to switch the colours of IRC and mail (although mr_tom's insta-goth key will do the trick nicely). They're not as important as Word, since I can just pop inside if I need to look at them. Word is the important thing, since I'd like to be able to work on the balcony without straining my eyes. I only mentioned colours in case it helped define the problem a bit better.
Edited again to add: I seem to be making a complete mess of explaining what I want to do. I can work in Word for a bit, but it hurts my eyes because the white bits of the screen look so dark. I've just found the contrast slider, so will try that next time we get some sun.