How can I make GMail messages appear w/ white text on black background?

Maybe this https://chrome.google.com/webstore/detail/inverter/ledjglfajmbclcmlohelooocdmnfdcci?

The https://addons.mozilla.org/en-US/firefox/addon/nosquint/ extension for Firefox allows you to override site text and background colors for any site without needing to edit CSS, as well as handing per-site text and overall zoom levels in a completely fuss-free manner. Might be worth switching browsers for, especially since Firefox's text rendering is noticeably better than Chrome's to begin with.

I don't know the answer to that particular question but a trick I use when I'm having trouble reading the text is to click once in the region where I'm reading and press Ctrl-A, which usually causes all the text to be highlighted, which on Windows usually makes it white on a blue background. You may also know of Ctrl-plus and Ctrl-minus, which should zoom in and out on most web sites. (But which may cause a horizontal scroll bar to appear, so depending upon how dexterous he is with the mouse it might be more trouble than it's worth.)

Since I don't know of any stylish/etc. themes, I offer an alternative until you find one: Firefox offers the ability to force font and background colors as a normal option. Go to Options, Content, then click the "Colors" button: select black font, white background, then uncheck the box that says something like "let websites choose their own colors." I use this for font forcing a lot.

He's on PC. Unfortunately, can't switch to FireFox, since he's used to Chrome and very good at using a Chrome text-to-speech browser extension to have his e-mails read aloud. The inverter extension for Chrome looks promising, I'll try that!

Depending on his OS, there are ways to invert everything on Windows as well, although it's a little more involved than a keyboard shortcut. Just google "inverting colors on windows [OS]" and look for the right tutorial. He probably would benefit from a high contrast color scheme, as well.

It might be a bit of a learning curve but you could always set up his gmail to POP enabled, then use a mail client like Thunderbird (or another program) to read his mail. Display is changed by going to Tools > Options > Display -- you can set it to black background, white text very easily. You can also change the font size. He should still be able to use Chrome and log in gmail to access any emails he wants read aloud-- you can set up gmail to keep copies in the inbox, although Thunderbird may have an text-to-voice extension to do that too. Alternatively, as XMLicious suggested, ctrl + is really handy.