Does somebody know, how to dim the display light of the radio?

In my opinion it is very bright when you drive at night...

The manual (german...) says, the function will appear in the OPT - menu of the radio, when you swap to the end of the OPT-menu.
But the part where it is described and explained is missing in the manual and after the Phone-mute-menu there is nothing more....

Any help or advisories? Thanks!