Reinterpret HasDarkBackground as Relative to ForegroundColor

[erikgeiser]

Currently, HasDarkBackground() just checks if the lightness is smaller than 0.5. This is obviously subjective (as acknowledged in the docs with the work "dark-ish") but what's more important is that it is missleading for low-contrast color schemes where both the foreground and the background has a high or low lightness.

Another approach to HasDarkBackground() would be to compare the foreground lightness to the background lightness. This would be more in line with what most people think of when a color scheme or background is light or dark.

An implementation could look like this:

// HasDarkBackground returns whether terminal uses a color scheme in which the
// background is darker than the foreground.
func HasDarkBackground() bool {
    _, _, fgLightness := termenv.ConvertToRGB(termenv.ForegroundColor()).Hsl()
    _, _, bgLightness := termenv.ConvertToRGB(termenv.BackgroundColor()).Hsl()
    return bgLightness < fgLightness
}

This way, it can be used to implement color-scheme aware dimming as described in #45.

Alternatively, the functionality above could be added alongside HasDarkBackground() with the name HasDarkColorScheme().

I can submit a PR if you like this idea.

[BachoSeven]

I second this.

While using the glow CLI program, I noticed that my gruvbox-light terminal colorscheme was determined by termenv to be "light" (through the dark-mode detection code), which was very counter-intuitive for a feature which is supposed to automatically detect whether the theme is dark or light.

This is an issue if you switch between light and dark themes daily, so that setting a theme in the config file and calling it a day isn't an option.