The adjective crepuscular describes anything that's related to twilight, like the crepuscular glow of the dimming light on a lake as darkness falls.
If the light outside is fading, you can call it crepuscular. Anything that resembles dusk, or happens at that time of day, is crepuscular, whether it's gloomy indoor light or the sky at twilight. Some animals are also crepuscular: they tend to be most active in the evening. The Latin root word says it all — crepusculum means "twilight."