Nokia C5-00 - How can I control the level of brightness on my Nokia C5-00 display?

Select MenuSettings > Settings > GeneralPersonalisationDisplay > Brightness  > Change and adjust the slider between Dark and Bright.

If you activate Power saver directly from the Power key, the brightness is set to minimum by default and cannot be adjusted.  To deactivate, press Power key > Navi up > Deactivate Power saving.