CLI TUI composer has no background fill over SSH/tmux while diff backgrounds render correctly

What version of Codex CLI are you using?

• Codex CLI: 0.146.0
• Installation: Linux x86_64 musl binary on the remote host

Environment

• Local terminal: Windows Terminal on Windows
• Remote host: Ubuntu 22.04 x86_64
• Connection: OpenSSH with ssh -tt, then Codex running inside tmux
• tmux: mouse on, default-terminal=tmux-256color
• Environment passed to Codex:
• TERM=xterm-256color
• COLORTERM=truecolor
• TERM_PROGRAM=WindowsTerminal
• WT_SESSION is present
• NO_COLOR is unset

What issue are you seeing?

The Codex CLI TUI composer/input area has no distinct background fill when running through SSH and tmux, even though terminal color capability detection is correct.

The diff renderer is able to show green/red background highlights for added and deleted lines. Foreground colors and the status line also render correctly. However, the welcome/session panels and the composer input row remain transparent and use only the terminal's default background.

This is a reproducible Codex CLI rendering issue over SSH/tmux, rather than a general request for more theme options.

Changing the Windows Terminal profile between light and dark does not change the composer background. Setting tui.theme also does not change this area.

Steps to reproduce

1. On an Ubuntu 22.04 x86_64 host, install Codex CLI 0.146.0.
2. Open Windows Terminal on the local Windows machine.
3. Connect with SSH using a TTY and attach to or create a tmux session.
4. Export the terminal metadata shown above and run codex.
5. Display a change so that the diff renderer shows added/deleted lines.
6. Compare the diff area with the composer/input area at the bottom of the TUI.

Expected behavior

The composer/input area should have a documented, readable semantic style, including a configurable foreground/background pair or a built-in dark/light/high-contrast theme. At minimum, the behavior should be consistent with the TUI's diff background rendering when truecolor support is available.

Actual behavior

• Diff backgrounds render correctly.
• Foreground/status colors render correctly.
• The composer/input area has no distinct background fill.
• The composer is difficult to distinguish from surrounding terminal content when semantic contrast is needed, regardless of the Windows Terminal profile.
• Switching the terminal profile or tui.theme does not resolve it.

A tmux capture-pane -e capture shows truecolor foreground sequences and diff background sequences, but no background sequence on the composer row. This suggests that the composer is not emitting a background style, rather than tmux or Windows Terminal dropping it.

Related issues and context

• Related to #21130, which requests configurable semantic colors for the main TUI, including the composer, borders, status line, and selections.
• PR #13016 fixed Windows Terminal diff truecolor detection, but explicitly does not cover the chat widget or composer.
• This report is specifically for the Codex CLI TUI over SSH/tmux, not the VS Code extension diff view.