The light is probably one or more LEDs soldered to the board and not adjustable. It seems to turns on as the ambient light drops (mine turns on as I ride into the shed, which is not really dark) so you might be able to cover the hole that it's reading daylight through to get it to turn on earlier.
And if the bike is a painted one with the traditional speedo, lots of people have trouble with the low contrast. It was the same on my Scout. My 2018 Springfield DH has the black speedo and it's no trouble to read. Some people have swapped out their speedo, but it means the odo goes back to zero.
I am 59 and have aviation style bifocal readers for night. Straight glass looking ahead but the bottom is I believe a 1.5 reader. I can read the Speedo and MFD just fine. It’s mostly a blur at night without the readers but I know 60 mph is with the needle at 12 o’clock.