Adaptive Theme Selection Based on the Terminal Color Scheme

[luca-schlecker]

Fixes #8899, #10281

Overview

Introduce theme = { light = "<name>", dark = "<name>" } syntax inside config.toml to allow adaptive theme selection based on the terminal color scheme.

Theme selection is based on terminal-colorsaurus which has also been used in delta and bat.

Considerations

• If the theme detection fails, dark mode is chosen by default.
• The new syntax using an untagged enum ensures existing configs remain valid.
• Theme validation (ensuring the theme name exists) is only done for the detected theme. This would defer errors if a missing theme is specified but reduces code changes.
• terminal-colorsaurus' code is only called in case an adaptive theme has been specified, guaranteeing practically zero overhead for static themes.