[Griffinaz]

If there was a means to turn off the clock then you would notice an increase in battery life. Anything running 24 hours a day uses a part of the battery life. Since there is no means to turn it off then you can't tell how much it is taking from the battery, but I can guarantee you that the clock does drain battery life.
To run a program that monitors that time to change the theme is actually adding 2 things. You would be adding an application that monitors the time and also adding a program launcher that sits waiting to be told to go ahead and launch yet another application at XX:XX time. These will drain the battery faster.