What are we talking about? It is impossible to have a GIP script that could be COMPLETELY modified with CSS, so that it could fit ANY website, unless you have a lot of time and full knowledge of the possibilities of CSS, and that each and every used tag has attributes class and ID set...
Next, a lightweight GIP script would mean one with set layout, possibly only the colors and borders and such could be changed with CSS, which means no site integration, only basic styling and features.
A good GIP script is one modified for the needs of ONE side, instead of having it either hard to style or hard to integrate with your site. So I'd say you'd just get a GIP script with as many features as you want and edit it, add counting uptimes to it, style it, integrate it, use it. Go Notepad.