As @gr5 said, you go to settings under the 3rd icon on the touch screen (its a cog icon) and pick settings and then scroll down to frame light, You can also dim the LED display screen also, i dim both to the min setting when the printer is left running over night.
On the s5 start in the bottom menu thing (the bottom of the left 3 tabs).
It's under "frame light".
You can dim it while printing. I've done it many times.
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