Sites that use the “960 Grid System” are automatically detected; click the “960” icon in the address bar to overlay a grid over your webpage.
Auto-detection is triggered when the CSS classes container_12, 16 or 24 are found.
The colors can be changed using a popup (disabled by default); to activate it, right-click the “960” icon and choose “Options”.
Details about the “960 Grid System”: http://960.gs