Support custom theme colors / respect terminal ANSI palette

Problem

Claude Code's /theme light and /theme dark use hardcoded RGB colors for code blocks, diffs, and UI elements. These don't respect the terminal's ANSI color palette, so custom terminal themes (e.g. sepia/warm-toned schemes) end up with clashing code block backgrounds and diff highlights.

Specifically:

• Diff context lines use hardcoded dark backgrounds even in light theme
• Code block backgrounds don't map to ANSI colors
• No way to customize or override these colors

What I'd like

One or more of:

1. Respect terminal ANSI palette — use ANSI color codes instead of hardcoded RGB so terminal themes control the appearance
2. Custom theme file — allow a ~/.claude/theme.json (or similar) to override specific colors (code block bg, diff added/removed, diff context, etc.)
3. More built-in themes — a "light-warm" or "sepia" variant that uses warm tones instead of cool grays

Context

I use a custom sepia/parchment terminal theme (Catppuccin Latte base with warm overrides in wezterm). The /theme light switch fixed the worst issues, but code blocks and diff context lines still use colors that don't harmonize with warm-toned terminal backgrounds. The ANSI overrides I set in my terminal config are bypassed because Claude Code emits true-color (24-bit) escape sequences.

Environment

• Claude Code CLI (latest)
• wezterm on macOS
• Custom color scheme (sepia variant of Catppuccin Latte)