Large LED Array powered via USB
LEDs will produce light well below 20 mA. The backlight of our display uses a LED with around 1 mA current - you can't see it in daylight but when it's dark it's fine.

You have around 5 mA per LED, that should be plenty (no idea what a teensy is and how much current it needs). Modern LEDs are almost blinding at 20 mA. However I'm not sure if you will find modern and efficient LEDs in through hole, so I'd use SMD LEDs.