[FEATURE] Allow users to customize dark mode color palette (e.g. dark blue instead of muddy brown)

Summary

The current dark mode color palette uses warm muddy brown/tan tones that many users find unpleasant. Please let users override the dark mode palette — at minimum offer a preset alternative (e.g. dark blue), and ideally allow custom color values via settings.

Motivation

• Color preference is subjective; the current dark mode tones feel "mud brown" to me and likely to others.
• Other terminal tools (and Claude Code's own theme overrides via #52465) hint this is already partly possible, but not exposed as a first-class option.
• Lets users match Claude Code's appearance to the rest of their terminal/editor setup.

Proposed UX

Two options, in increasing order of effort:

1. Preset themes: Add a "Dark Blue" (and maybe "Dark High Contrast", "Dark Solarized") option alongside the existing dark theme in /config.
2. Custom palette: Allow users to set individual color slots in settings.json — primary text, accents, borders, diff add/remove, etc. — and have them reliably apply across all UI surfaces (see #52465 for an existing gap).

Related

• #52465 — custom theme overrides silently ignored in some slots
• #53154 — soften pure-black background on macOS dark theme
• #50086 — system-following theme option
• #48158 — dark mode redesign feedback