Have you ever noticed that there are lots of secrets that are not shown on the instructions? I have just found one for all those people who think their tamagotchi screen is too bright or too dark.
First you press B to get you to the time screen. Next you must press B and C at exactly the same time and the screen will then say CONTRAST and to lower the contrast to make the screen brighter, press A. To increase the contrast and make the screen darker, press B. When you are happy, press C and you will return to the time screen.
Hope it helped!