Improve user message contrast in the CLI TUI

What variant of Codex are you using?

CLI

What feature would you like to see?

User messages in the CLI TUI can be hard to distinguish from Codex responses when the terminal theme renders both with nearly the same foreground and background colors.

A small but high-impact improvement would be to render user messages with a stronger background panel so they remain easy to visually scan in the transcript.

A good default would be:

• strengthen the user-message background blend when the terminal background is known
• use an explicit gray fallback panel when terminal background detection is unavailable
• apply the same behavior in both codex-rs/tui and codex-rs/tui_app_server

This is narrower than full theme customization, but it materially improves readability immediately.

Additional information

This seems related to #2020, but it is a more targeted usability fix that does not require a full light/dark theme system.

I have a working branch with the change here:

• de-monkey-v:gyu/tui-user-message-contrast

The branch updates:

• codex-rs/tui/src/style.rs
• codex-rs/tui_app_server/src/style.rs
• related TUI snapshots

Local validation for that branch:

• just fmt
• cargo test -p codex-tui --lib
• cargo test -p codex-tui-app-server --lib
• ./tools/argument-comment-lint/run.sh

If useful, I can also turn that branch into a patch or follow any preferred contribution path for repos with collaborator-only PR creation.