how do i connect my windows phone to home theater system over bluetooth?

You need a Bluetooth enabled amp that has a bluetooth receiver system in it, then follow the manual that comes with the unit.

If your system isn't Bluetooth capable, you can buy a Bluetooth receiver on eBay for around $10. Then you would just plug that to your sound system, and Bluetooth stream to the receiver.

If the system is BT - capable,
simply Read The Owner's Manual
and find out how to make it connect.

If it isn't BT - capable, get an add-on BT receiver
and connect that to a line-level (aux) input on the system,
then pair the phone to the BT receiver ---
once again, Following The Included Instructions.