An edit of http://freestyler.ws/style/98438/racket-docs-dark with darker fonts and some better backgrounds for certain elements.

Caveat: this style works better in WebKit-based browsers, such as Google Chrome or Safari, as it uses a WebKit-only property to darken some elements.

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Note 1: if you want to apply this style to your local docs, simply find the doc directory that is under your main Racket directory (invoking $ which racket in your shell may aid in finding this) and copy this CSS to the bottom of doc/manual-style.css. I'm sure you could write a script to do that automatically, if you're into that sort of thing ;) (fyi, the CSS file http://freestyler.ws/style/137889/racket-docs-darker in case you are interested in that)

To also apply this on your local package docs, search page, etc. you may also have to patch user-specific doc/manual-style.css files that are located in another directory somewhere under your home directory. On OS X, this is ~/Library/Racket. Linux probably has a similar location somewhere in your home directory, as well. For Windows, I'm not sure.
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Note 2: this style also applies to http://jeapostrophe.github.io as well as http://www.greghendershott.com/fear-of-macros/ It's applied to these sites because 1) they use the same style as the Racket docs and 2) they contain a lot of useful information about Racket, so they're pretty much Racket docs as well.