Can you turn off the instrument panel lights?
It depends. 

On my Cherokee, when you turn the navigation lights on you can push the switch up or down. Up gives you bright lights on the panel and down gives you dim lights. There is also an overhead red light that has a sliding door that you can open or close for more or less light. There is one rheostat that controls the lighting in the VOR/LOC indicator.

On my Cessna 210 there are lights in some of the instruments, post lights next to the instruments, and overhead lights. The lights are controlled with two switches and several rheostats. The GPS has independent lighting controls.

The Piper Arrow has two rheostats—one for navigation and radios and one for instrument panel lights. If you turn the navigation lights on there is always some lighting on the radios. The GPS has its own lighting controls.

My friends Piper Dakota has lights for the instruments as well as a light strip along the base of the glare shield that illuminates the entire panel. Controlled by a combination of switches and rheostats.

Older general aviation aircraft have widely varying approaches to nighttime lighting so before you fly at night you need to become familiar with the idiosyncrasies of each one.