Since there are different keyboards with different Jumper models and some like my new 3S has no brightness control, here is a solution what works well. The 3S volume control is on Fn F3 / F4.
So with this little gem I hit Winki (Windows key) twice and then Fn +F3 makes the screen darker.
https://www.reddit.com/r/Surface/comments/3h3dt8/change_screen_brightness_from_the_volume_keys/
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1981 soldered my first Sinclair computer 1K, tapedeck * 1984 build and sold IBM clones 8Mhz, 512K, 20MB HDD * 2018 messing with ultrabooks