[FEATURE] Configurable diff background colors / support terminal theme colors

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The current diff rendering uses hardcoded bright green and red background colors for added and removed lines. In dark mode terminals, these colors are visually jarring - they neither match the terminal's color scheme nor adapt to the user's configured theme palette. The high contrast between the bright diff backgrounds and a dark terminal creates significant eye fatigue during extended sessions, making the tool uncomfortable for daily use.

<img width="1721" height="1055" alt="Image" src="https://github.com/user-attachments/assets/2d778e46-9a68-4618-960f-094a826c4273" />

Proposed Solution

Add a diffStyle setting to settings.json with three options:

• "highlight" (default) - current behavior, bright background fill on added/removed lines
• "theme" - no hardcoded RGB; use ANSI color indices 1/2 (terminal-defined red/green) so diff colors inherit from the user's terminal palette
• "minimal" - no background fill at all; color applied only to the line number and +/- symbol, body text remains default foreground

  {
    "diffStyle": "minimal"
  }

This requires no new dependencies and no layout changes - only the set of ANSI attributes applied per diff line changes between modes.

<img width="1721" height="1055" alt="Image" src="https://github.com/user-attachments/assets/d0a45bb3-81a0-4a11-aff9-6e8927c82cc0" />

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

Interactive mode (TUI)

Use Case Example

_No response_

Additional Context

_No response_