I wrote about this problem to the support service.
Here’s what they answered:
"We are aware that some users find the LED light too bright, and implementing a personal setting for setting your own preferred LED brightness level is on our to-do-list. The Octatrack MKII was recently the first product with these button LEDs that got a brightness setting and the plan is to have it on all products as soon as possible. Hopefully for the next OS update, but I can’t promise anything.
Something you could do in the meantime if this really bothers you is to remove each button cap and put a tiny piece of paper on top of the LED (to block some of the light) before putting the button cap back in place. Use a thin plastic tool (like a guitar pick or credit card) to remove a button cap. Insert the tool in the gap between the panel and the button cap, then gently heave to dislodge the cap. To put the button cap back, just press it in place. This should be completely safe to do."
And I did it
It turned out to be very simple. It took two hours for all the buttons.
The main thing is to use a soft guitar pick.
And instead of paper, I used shrink tubing cut into squares.