The current OpenSprinkler runs with 24v AC, so the lighting outside needs to be compatible. From my understanding, the new 3.x version of OpenSprinkler will possibly run on DC voltage, and that means you can easily power some really low lighting, as long as you’re not running the sprinklers at the same time, the voltage needed may dip too low for one or the other.
But yeah, look for 24v AC lighting if you want to power it, and remember, you’re capped at 1A of power, for your OpenSprinkler and your lighting, and the power up of the lighting may require more amperage too.