Dimming LED Strip Via Motherboard Header

I have two new Darkside G2 LED strips that I have connected from a 3pin splitter to my Asus motherboard via 3pin header. I noticed that I am unable to control the brightness because there is no RPM signal available for Fan Xpert to adjust from. I tried attaching a 12v-7V step down adapter from a Corsair SP120 fan, but the brightness is still the same as if it was running a full 12V. Is there any way around this?