The easiest option if you're working digitally and in color is pretty much just to create a new layer over top of everything else (except text/speech bubbles), set it to multiply, and dump in a dark blue or blue grey or whatever suits your fancy. Adjust hue and layer opacity until you get something that looks good.
Obviously you can get more advanced than that with it; sometimes I use multiple layers on different settings to get a certain tone. Honestly it's just something you need to play around with until you find something that works for you.
Sometimes a night sky and some moonlight can also help you out; here's some of my night-scene art:
This technique also works pretty well for other lighting scenarios; I used a some grey layers to desaturate and darken this scene for a rainy look: