This has only been tested in the Android 2.3.4 stock browser. To use it, copy the code from the page linked to below into the clipboard and add a new bookmark, pasting the code into the URL box. Give it a name like "Invert Color". Then when you're viewing a page you want to invert, open the bookmarks menu, and click the bookmark.
You still have to click the link for every page you browse to for now, but perhaps somebody knows a better way. I did try to contact Dolphin for info on writing Addons, but it doesn't seem very open. I haven't used firefox mobile too much yet, but maybe there is potential there...
I wasn't sure exactly where to post this, and since it might work on any device with a browser that supports bookmarklets, i put it in general. mods should feel free to move it with my thanks!