The power is directly from USB, so the voltage is +5V.
It would be better to take the power and ground from pins on the PCB than from the jack. Do not take it from the Teensy - its lines are thin.
First, check how much power your LED strips are going to need. Then check how much power that can be supplied through "full-speed" USB 1.1 and subtract what the components are drawing. You could assume that the AVR and I/O expander are drawing the "max" of what it says in the data sheet plus a little bit more.
Backlit keyboards have low-power LEDs, and they are also usually strobed so that only a handful of them are actually lit at once, but it is done so fast that it appears to the lazy human eye that all of them are lit at once.
LED strips don't always conserve power that way.
BTW... each USB client is supposed on startup/reset to tell host how much current it is going to need, max. I think that the usual firmwares ask for the max amount, but it wouldn't hurt to check the source code for the firmware that you intend to use.
