[FEATURE] No theme token for the diff panel background

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

Summary

The diff panel paints its own background, and no theme token controls it. On a dark base it
fills with a fixed dark neutral that has no relation to the active theme or to the
terminal's background.

Detail

The color token reference in terminal-config covers added/removed line colors
(diffAdded, diffRemoved, the *Dimmed and *Word variants) and three transcript
message backgrounds (userMessageBackground, bashMessageBackgroundColor,
memoryBackgroundColor), but nothing for the diff panel's own canvas. Grepping the 2.1.272
binary for background, panelBackground, secondaryBackground, diffBackground, and
canvas turns up no such token either.

Confirmed by capturing what the pane actually emits (tmux capture-pane -p -e): the panel
fill is an explicit background color on ~149 cells, distinct from the diff line colors and
distinct from 49 (terminal default). Cells the panel doesn't paint fall through to the
terminal default correctly — it's only this explicitly filled region that can't be themed.

The result: on a terminal with a tinted background (e.g. Dracula's #282a36), the panel
reads as a near-black rectangle inside an otherwise themed UI, and no custom theme can
change it.

Request

Expose a token for the diff panel background, or leave those cells at the terminal default
(49) so they inherit the terminal's background like the rest of the transcript.

Environment

• Claude Code 2.1.272, macOS 15 (Darwin 25.5.0), tmux 3.6b, Alacritty

Proposed Solution

should be configurable in theme settings

Alternative Solutions

none.

Priority

Low - Nice to have

Feature Category

Interactive mode (TUI)

Use Case Example

_No response_

Additional Context

_No response_