Select the View Menu.  The Night Lighting entry is a Toggle that sets night lighting on or off.  In this case Night Lighting is off, so clicking the menu entry will set it on.

This shows the menu entry checked and Night Lighting will be on.  Click it again to turn Night Lighting off.

The airport at night showing the lights.  Most objects can still be selected edited, moved and so on in this mode.

Pressing the 'L' key will toggle the day/night mode.  It will also set the check status of the menu item so the two methods are inter-linked.