Furuno LH-5000 Operator's Manual page 27

Press MENU key on the front panel.
Press ENT key to access the "DIMMER" interface on the screen.
Press Up or Down key to increase or decrease the brightness of the
keys on the front panel.
Press ENT key to confirm your settings.
If you do not adjust the brightness within 60 seconds, the LH-5000
returns to the previous mode automatically.
Press the MENU key on the front panel.
Press the Up or Down key to select "CONTRAST" and press the
ENT key.
Press the Up or Down key to increase or decrease the contrast of the
screen.
Press ENT key to confirm your settings.
To adjust the volume from the internal or external speaker, rotate the Listen