Command 'xprintidle' returned non-zero exit status 1 in a systemd service

I have a Raspberry Pi with Raspbian Buster Lite which I start in KIOSK mode with autologin. The purpose is to display a single web page on the connected 7" touchscreen (with Chromium). If the system is idle, my python script should detect it and dim the screen automatically. When a user interacts with the screen again, it should light up.

This is my script (dim_screen.py):

#!/usr/bin/env python3
import subprocess
import time
import sys

from rpi_backlight import Backlight

backlight = Backlight()

# read arguments from the run command: 
# idle time before dim (in seconds)
idleTimeBeforeDimMS = int( sys.argv[1] )*1000

# brightness when dimmed (between 0 and 100)
brightnessDimmed = int( sys.argv[2] )
brightnessFull = 100

def get(cmd):
    # just a helper function
    return subprocess.check_output(cmd).decode("utf-8").strip()

isIdle0 = False
stateChanged = False
timeIntervalToWatchChangesS = 100 / 1000

while True:
    time.sleep( timeIntervalToWatchChangesS )

    currentIdleTimeMS = int( get("xprintidle") )

    isIdle = currentIdleTimeMS > idleTimeBeforeDimMS
    stateChanged = isIdle0 != isIdle

    if isIdle and stateChanged:
        # idling
        backlight.brightness = brightnessDimmed
    elif not isIdle and stateChanged:
        # active
        backlight.brightness = brightnessFull

    # set current state as initial one for the next loop cycle
    isIdle0 = isIdle

My script works as expected when I start it from a SSH (I can test it by connecting to my Pi from Putty on a Windows 10 computer): pi@raspberrypi:~$ /usr/bin/python3 /home/pi/startup_scripts/dim_screen.py 10 25

Unfortunately this is not the case when I try to run the script as a service on boot from systemd (or via systemctl start dim_screen.service). I can't get it to work.

This is my dim_screen.service file:

[Unit]
Description=Ensures that the 7" raspberry pi screen automatically will dim to 10% brightness after 25 minutes of user not interacting with it 
After=multi-user.target

[Service]
ExecStart=/usr/bin/python3 /home/pi/startup_scripts/dim_screen.py 10 25

[Install]
WantedBy=multi-user.target

If I do a reboot and run sudo systemctl status dim_screen.service, I get this:

I believe that the most important debug info is this line: Command 'xprintidle' returned non-zero exit status 1.

That's strange I think? Here is what happens if I run xprintidle in the terminal:

pi@raspberrypi:~$ xprintidle
11399922

In other words, I get a valid response from xprintidle. I believe there is something else that I do not understand?

What is the correct way to start my python script (dim_screen.py) as a service?