feat: add user-customizable color skins via YAML

[VrtxOmega]

Closes #2171

Summary

• New /skin slash command — switch between named skins at runtime; works like /theme but for user-defined palettes
• YAML skin loader — ~/.kimi/skins/<name>.yaml files define complete color palettes in a Hermes-compatible format; any omitted token falls back to the dark defaults
• Backwards-compatible — dark and light built-ins are unchanged; /theme still works; get_active_theme() returns "dark" for custom skins so existing callers aren't broken
• Config integration — skin = "<name>" in config.toml activates a skin on startup, same lifecycle as theme
• 50 new tests — cover dataclasses, YAML parsing (valid/partial/bad), filesystem discovery, public API, backwards-compat bridge, and all five color-resolver functions
• Docs updated — /skin section added to EN and ZH slash-commands reference; CHANGELOG entry under Unreleased

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"

Test plan

• All 50 new tests pass (pytest tests/ui/test_skin_system.py)
• Existing test_console_theme.py unaffected
• Manual: mkdir -p ~/.kimi/skins && echo "name: test" > ~/.kimi/skins/test.yaml, then /skin test in kimi shell
• Manual: verify /skin with no args lists available skins
• Manual: verify skin = "test" in config.toml activates on startup
• Manual: verify /theme dark still works after /skin switch

🤖 Generated with Claude Code

[devin-ai-integration]

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[Copilot]

Pull request overview

This PR introduces a new “skin” system that lets users load custom UI color palettes from YAML files under ~/.kimi/skins/ and switch them at runtime via a new /skin slash command, while aiming to preserve existing dark/light theme behavior for backwards compatibility.

Changes:

• Add skin dataclasses + YAML loader/discovery and make all UI color resolver functions skin-aware (src/kimi_cli/ui/theme.py).
• Add /skin slash command plus config integration (skin = "<name>") and shell startup initialization from config.
• Add tests for parsing/discovery/public API, update docs (EN/ZH) and changelog.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/kimi_cli/ui/theme.py	Implements skin model, YAML loading/discovery, and routes all color resolvers through the active skin.
src/kimi_cli/ui/shell/slash.py	Adds /skin command and persists config.skin.
src/kimi_cli/ui/shell/__init__.py	Initializes skin/theme from config on shell startup.
src/kimi_cli/config.py	Adds skin field to the config model.
src/kimi_cli/ui/shell/prompt.py	Adds /skin to toolbar tips.
tests/ui/test_skin_system.py	Adds new test suite covering skin dataclasses, YAML parsing, discovery, and resolver outputs.
docs/en/reference/slash-commands.md	Documents /skin usage and YAML structure.
docs/zh/reference/slash-commands.md	Documents /skin usage and YAML structure (Chinese).
CHANGELOG.md	Adds an Unreleased entry for the new skin feature.

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