Windows TUI missing shaded background for user input (no tint)

Summary
On native Windows terminals (PowerShell), the Codex TUI does not render the shaded background/tint for user input blocks and transcript items. The same UI on Linux/macOS shows a distinct background for user input ("chat box" look). Unicode box‑drawing characters render correctly, but the background color never appears on Windows.

Steps to reproduce

1. Run codex on native Windows (PowerShell).
2. Observe the input area and user prompt blocks in the transcript.

Expected
User input blocks and the input area should have a subtle shaded background (as seen on Linux/macOS).

Actual
No background tint on Windows; only text and borders are shown.

Environment

• OS: Windows (native)
• Shell: PowerShell (pwsh)
• Terminals tested: Windows Terminal, VS Code terminal, Alacritty (same result)
• Codex version: 0.79.0 (Windows screenshot)

Notes
This looks like a Windows‑specific rendering limitation in the TUI color pipeline: user_message_style() relies on terminal_palette::default_bg() to compute a blended background, but default_bg() returns None on Windows (default color query is compiled out). As a result, user message styles fall back to no background on Windows.

<img width="1426" height="547" alt="Image" src="https://github.com/user-attachments/assets/b647659c-6ca6-4446-a166-423b9afd8cba" />

<img width="1635" height="1177" alt="Image" src="https://github.com/user-attachments/assets/8b9684f1-3980-4628-a55d-cfd4ecb07cbb" />