The Fluke-120B series backlight adjustment is listed in the manual, below is a snap shot of that page.
What it doesn’t say is you don’t tap on the up or down arrow you have to hold down the arrow and it takes about 10 seconds before you see a change.
The default without anyone making a change is the brightest level but if you choose the down arrow and hold it for 10 seconds or longer the display will get darker and darker and if you use the up arrow the dark level will decrease up to the brightest level.