Simply drag the following link to your bookmarks toolbar. Then click it while viewing a site to invert the html colors. Note that this doesn't work on image colors.
This has only been tested in the Android 2.3.4 stock browser. To use it, copy the code 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 this bookmarklet.
Code
javascript:function loadScript(scriptURL) { var scriptElem = document.createElement('SCRIPT'); scriptElem.setAttribute('language', 'JavaScript'); scriptElem.setAttribute('src', scriptURL); document.body.appendChild(scriptElem); } loadScript('http://tonymuka.com/js/invert.js');