Allow customization of diff view highlight colors

Feature Request

When Claude Code displays file edits via the Edit tool, added lines are shown with a hardcoded light green background. On dark/black terminal backgrounds this creates a high-contrast block that can be visually jarring and is inconsistent with the rest of the terminal's color scheme.

Desired behavior

Allow users to configure the background color used for added/removed lines in the diff view, either via:

• A setting in /config
• A theme option that respects the terminal's color scheme
• An environment variable or config file entry

Current behavior

The diff highlight color is hardcoded and cannot be changed, even when the terminal background is opaque black.

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