This module examines the environment and selects an appropriate theme. THe current implementation is really basic. Future versions should look at more, partly OS specific, clues.

 theme(-Theme) is semidet[private]

Theme that suits the environment. Fails if we cannot tell, which leaves the default. Note that the default is a light theme, so failing here is the right thing on a light background.

 luminance(+RGB, -Luminance) is det[private]

Perceived brightness of RGB as a float in 0.0..1.0. This is the luma of the sRGB encoded components reported by the terminal, so green weighs a lot more than blue and adding up the components would call a saturated blue background light.