Support custom diff color themes

The hardcoded red/green diff colors in Claude Code's edit view clash with carefully chosen terminal themes (e.g. kanagawabones on Ghostty).

It would be great if /theme or a config file could allow customizing diff styling — particularly the added/removed line colors — to better match the user's terminal palette.

Current behavior: Diff colors are fixed regardless of terminal theme or Claude Code theme selection.

Desired behavior: Ability to customize diff highlight colors, either through:

• Extended /theme options
• A theme config file (e.g. ~/.claude/theme.json)
• Inheriting from the terminal's ANSI color palette