In addition to the Maxspeed style I posted earlier, Merkaartor now also has a style called "Lighting" that displays ways with the "lit=yes" tag in yellow, and "lit=no" in a dark grey. Once Merkaartor allows me to also select the canvas background color from a style definition, I'll switch to a dark background, giving something similar to a night mode.

If you don't run Merkaartor SVN, you can get the style from
http://trac.openstreetmap.org/browser/applications/editors/merkaartor/Styles/Lighting.mas and load it manually (Tools > Preferences | Style | (*) Custom).