That's the part for you 
http://docs.spark.io/hardware/#subsystems-power-regulator
Technically speaking, it's not really isolated and it needn't be, since it's both 5V and it shares GND.
One thing the docs above don't mention is,that you could power the Core via the 3V3 pin too, but since this is somewhat hairy it might be good it's not disclosed too prominently
Hi there, have you solved the problem and get your Neopixel ring working in the end? cause I have just the same issue, and Iām not quite sure it is about the power supply, because with the same code and same power supply i got my neopixel ring worked twice than i tried it with a much complicated code it didnāt light up, then i flashed it back with the code which was working before, it didnāt light up eitherā¦iām so confused and tried it for a few days, it still keeps dark, so frustratingā¦the neopixel ring i have is definitely working well, i have tried it with my arduino nano and it is shining brightly. if you have the problem solved please let me know how, thanks a lot!
@leaphr, One thing different between the Core and the arduino,is the voltage on the output pins ( 3.3 vs 5 volts). I have seen some indications that the neopixel wants 3.5 volts to indicate a high state, so the 3.3 volt may be a bit shy.
Some suggest a level shifter, or maybe reduce the voltage on the power supply for the neopixel a half volt or so. Since they suggest .7 times the power supply voltage.
5V * 0.7 = 3.5
4.5v * .7 = 3.15
What do you think?