The idea is simple... take the hour, minute, and second of the current time and use those values combined as the page background's hex color (#hhmmss).
Admittedly, there's not a lot of business purpose here, but it can be a good development exercise to use as an introduction to re-rendering page components using actionPollers and a few various Visualforce functions.
Can it be done? With the exception of the minimal polling time being 5 seconds, you know it!
Here's the end result:
Here's the Visualforce page: