LEDs
Stemi shield has an onboard LED connected to pin IO12 and can control external LEDs connected to GPIO pins.

Library
For simple usage DigitalIO can be used to turn it on or off, but if you want more precise control you can use PWM.
Example
digitalio_example.py
import board
from stemi.digitalio import DigitalInOut, Direction
led = DigitalInOut(board.IO12)
led.direction = Direction.OUTPUT
led.value = 0 # off
time.sleep(1)
led.value = 1 # on
time.sleep(1)
pwm_example.py
import board
from stemi.pwmio import PWMOut
led = PWMOut(board.IO12, duty_cycle=2 ** 16 - 1, frequency=50)
led.duty_cycle = 0 # 0% duty cycle (off)
time.sleep(1)
led.duty_cycle = int((2 ** 16 - 1) / 2) # 50% duty cycle (half on)
time.sleep(1)
led.duty_cycle = 2 ** 16 - 1 # 100% duty cycle (on)
time.sleep(1)