How to change the brightness of screen colors on Ubuntu?

Question

linux

I used to use Ubuntu 18.04 on a laptop, and there the colors were (I don’t know how to say it right) too bright, that is, the white color was very bright, so the white block on a slightly gray background was almost invisible.

I then sinned on the laptop screen.

Now, I rolled Ubuntu 20.04 onto my PC, and again the same thing.

White color simply merges with gray, etc., as if the screen brightness was turned up to 100%, which already illuminates everything.

You say "turn down the brightness of the monitor / etc", BUT! If I boot from under Windows, everything is in order with the brightness, as usual, I boot from under Ubuntu, and again everything is bright.

How to reduce the brightness to normal, it is very straining on the eyes.
I'm still silent about the twitching of the horizontal strip across the middle of the screen when you scroll...