[FEATURE] Let the `auto` theme switch between a custom light theme and a custom dark theme

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Since 2.1.111 the auto theme follows the terminal's light/dark background, which is exactly right for people whose terminal follows the OS appearance (macOS "Auto" appearance + iTerm2 with separate light/dark colors).

But auto can only choose between the built-in light and dark presets. It never selects the daltonized presets, and custom themes in ~/.claude/themes/*.json have a single base, so they can never take part in the switch either.

That makes the two features mutually exclusive: any custom color (for example a different userMessageBackground, or simply wanting the daltonized variants) means giving up automatic switching. The workarounds don't really work:

• a running session does not pick up a theme change written to settings.json from outside (observed on 2.1.263 and 2.1.266), so an external appearance watcher only affects sessions started afterwards;
• switching by hand with /theme every morning and evening, in several sessions that run 10+ hours across sunset, is tedious and easy to forget.

Proposed Solution

Let auto switch between an explicit light/dark pair. Any of these shapes would solve it:

1. Settings keys next to "theme": "auto", e.g. "themeLight": "custom:my-light" and "themeDark": "custom:my-dark", defaulting to light / dark as today. Built-in names such as light-daltonized / dark-daltonized should be accepted too.
2. A custom theme file with a light and a dark section (each with its own base and overrides) that auto switches between.
3. Smallest version: allow "base": "auto" in a custom theme, applying the same overrides on top of whichever built-in theme auto resolves to.

Switching should stay live, as auto is today, and /theme should list such themes.

Alternative Solutions

• /theme by hand twice a day.
• A launchd/cron job that rewrites theme in settings.json when the OS appearance changes: only new sessions get it, running ones keep the old theme.
• The light-ansi / dark-ansi bases, which take colors from the terminal palette that iTerm2 already switches: the 16 ANSI slots are shared with every other program, so repurposing one for a UI background breaks other tools.

Priority

Medium - Would be very helpful

Feature Category

Configuration and settings

Use Case Example

1. macOS is set to Auto appearance; iTerm2 uses separate light and dark colors and follows it.
2. I want a light-blue background behind my own messages in the light theme (the built-in gray is low contrast for me) and the default dark theme at night.
3. With this feature I would set themeLight: custom:light-blue and themeDark: dark once, and Claude Code would follow the terminal all day, including in sessions that stay open across the switch.

Additional Context

• Claude Code 2.1.266, macOS 26, iTerm2 3.6.11.
• Related: #55569 (custom user-message background, now possible through custom themes but not together with auto), #39369 (respect the terminal ANSI palette), #11813 / #16769 (auto theme, shipped in 2.1.111).