I also tend to change my theme during the day . Not sure that I would use a plug-in for this myself but I certainly encourage your enthusiasm.
It would be possible (I suppose..) to use geo-location-ing to find the sunset/sunrise times, and even the weather, for the user's location.
Some modern computers use "ambient lighting". I have no idea how this works but, presumably, if the computer is able to determine the *lighting *of the current environment, then it must (presumably) store this information somewhere.. in which case it could also be read (somehow?). This should fill your weekends