CartoType now has a night mode which can be turned on and off using the new SetNightMode function.
In night mode, everything gets darker except roads, which get brighter, giving a calmer appearance but maintaining contrast and readability.
Night mode works with existing style sheets, by blending in the current night mode color. The standard night mode color is a dark blue (specifically "ncsblue+0.75black" in CartoType color terminology), but you can choose your own using SetNightModeColor. You should use a dark color but that's not enforced. Icons are drawn in monochrome.
In night mode, the style sheet variable "night" is defined, allowing you to program different behavior. For example, minor roads could be suppressed at a different scale threshold.