Hi nikthomsen and welcome.
The switch for the backlit keyboard is an ambient light sensor next to the Speed/Stamina switch and this cannot be adjusted. You can only have the backlight on when it is dark (unless you cover the sensor) - but you can disable it altogether. You can also adjust the time before the light turns off and if you want it 'always on' when dark, choose the last option "Do not turn off (always on when dark)"
If you go to Vaio Control Center you will find these options under 'Backlit Keyboard'.

