Manual is linked in the article. From this:

"Display settings
Adjusting the display brightness
You can adjust the brightness of the display while the Siesta is in operation or in standby mode.
standby mode. To adjust the brightness of the Siesta display:
1. press the Menu button.
2. use < > to scroll to "DISPLAY" and press the Select button .
3. use < > to scroll to "BACKGROUND" and press the Select button.
4. use < > to scroll to "ACTIVE" and press the Select button to select the settings valid when the
change the settings valid when the Siesta is switched on. You can also scroll to < >
"STANDBY" and press the Select button to change the settings that apply in standby mode.
change the settings that apply in standby mode.
5. scroll to "LEVEL" with < > and press the Select button.
6. scroll with < > to select a brightness setting between "1" and "5".
7. press the Select button to confirm the brightness setting"

The brightness is automatically adjusted depending on the surroundings. In the dark, the display is very dim. If you turn the display away from you, you won't even notice it.