How do I activate the Day and Night changing theme on the Sony Ericsson T707?

Hi, I just bought a new Sony Ericsson T707 phone a few days ago. Strangely, the day and night changing theme on my phone doesn't seem to activate automatically even though I read that it will.

Please help me.

Thanks to all (:How do I activate the Day and Night changing theme on the Sony Ericsson T707?
it is automatically, perhaps you need to go to the service center before your guarantee expire