I was looking for a solution to this too and didn't find one i liked. Dolphin HD's "Reader" addon was close, but it added padding to the browser on my Epic.
This has only been tested in the Android 2.3.4 stock browser. To use it, copy the code from the link below into the android 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 this bookmarklet.
/ (sorry, i dont have enough posts here to link. check me out over at xda if you like...)