Support mode 2031 dark/light mode detection

[the-mikedavis]

Fixes #13281.

For better compatibility in detection we can also look at querying OSC 11 for the terminal's background color and trying to detect its perceived lightness (also see #12238). That will take a bit more work to add OSC 11 to Termina and then to handle the response.

[colemickens]

Tested! Works with ghostty!

[loralb]

Just a NIT: the option no_preference, while it makes total sense in the code, may be confusing for the user (i.e. why would I set my theme to not have a preference?). What do you think of using something like default or fallback instead?

[the-mikedavis]

It's borrowed from Kitty's naming https://sw.kovidgoyal.net/kitty/kittens/themes/#change-color-themes-automatically-when-the-os-switches-between-light-and-dark

fallback does sound nicer to me though

[cor]

I've been running this for a week and it works flawlessly!

Very nice UX improvement

[Nico-Mayer]

Hey, I tested this using a Helix build from source and Ghostty on macOS, but Helix seems to always default to the dark theme regardless of my system appearance settings.

Helix config (config.toml):

[theme]
dark = "catppuccin_frappe"
light = "catppuccin_latte"

Ghostty config:

theme = dark:Catppuccin Frappe, light:Catppuccin Latte

In Ghostty, the theme correctly switches when toggling the system appearance, but Helix always uses the dark theme regardless of the system setting.

[loralb]

@Nico-Mayer look at #14534