This feature is available in the stock browser if you are running Cyanogen 6.1. You must be rooted and have a phone that has a Cyanogen ROM build available. Look under browser setting for "Invert Color".
Honestly, if you haven't rooted your phone, do it. Once you are rooted, nothing appears any different. However, you have many new options and features available to you on demand, including speed enhancements, UI enhancements, extra features, and regular updates.