RFC: user-customizable color skins via YAML (~/.kimi/skins/)

Problem

The current /theme command offers two built-in palettes (dark / light). There's no way for users to define their own color scheme without patching source code. Power users running custom terminals, branded environments, or accessibility-tuned setups have nowhere to go.

Proposed solution

Add a YAML-based skin system that lets users drop a file into ~/.kimi/skins/<name>.yaml and activate it with /skin <name> or skin = "<name>" in config.toml.

Key design decisions:

• Hermes-compatible format — same colors: / branding: / font: structure, so skins are portable across compatible tools
• Backwards-compatible — /theme dark|light still works; get_active_theme() returns "dark" for custom skins so no existing callers break
• Graceful fallback — any token omitted from the YAML falls back to the dark-theme default, so a minimal skin file is just a few lines
• Zero new dependencies — pyyaml is already in the dependency tree

Example skin file

name: dracula
description: Dracula color scheme
colors:
  diff_add_bg: "#1e3a2e"
  diff_del_bg: "#3a1e2e"
  mcp_connected: "#50fa7b"
  toolbar_yolo: "#ff79c6"
branding:
  prompt_symbol: "❯"
font:
  primary: "Fira Code"

Implementation

A working implementation is ready in PR #2170. It includes:

• Rewritten theme.py with Skin / SkinColors / SkinBranding dataclasses
• _load_yaml_skin() and _discover_custom_skins() for filesystem loading
• /skin slash command (list / switch)
• skin field in config.py
• 50 passing tests covering all code paths
• EN + ZH documentation

Happy to adjust the design based on feedback before merge — opening this issue to start the conversation per the contribution guidelines.

Start by reviewing PR #2170, which contains the working implementation described in this RFC. Inspect theme.py, config.py, the 50 associated tests, and the EN/ZH documentation; the work is complete when custom YAML skins, /skin switching, configuration support, fallback behavior, and compatibility requirements are covered and the tests pass.