update composer + user message styling (#4240)

Changes:

- the composer and user messages now have a colored background that
stretches the entire width of the terminal.
- the prompt character was changed from a cyan `▌` to a bold `›`.
- the "working" shimmer now follows the "dark gray" color of the
terminal, better matching the terminal's color scheme

| Terminal + Background        | Screenshot |
|------------------------------|------------|
| iTerm with dark bg | <img width="810" height="641" alt="Screenshot
2025-09-25 at 11 44 52 AM"
src="https://github.com/user-attachments/assets/1317e579-64a9-4785-93e6-98b0258f5d92"
/> |
| iTerm with light bg | <img width="845" height="540" alt="Screenshot
2025-09-25 at 11 46 29 AM"
src="https://github.com/user-attachments/assets/e671d490-c747-4460-af0b-3f8d7f7a6b8e"
/> |
| iTerm with color bg | <img width="825" height="564" alt="Screenshot
2025-09-25 at 11 47 12 AM"
src="https://github.com/user-attachments/assets/141cda1b-1164-41d5-87da-3be11e6a3063"
/> |
| Terminal.app with dark bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 45 22 AM"
src="https://github.com/user-attachments/assets/93fc4781-99f7-4ee7-9c8e-3db3cd854fe5"
/> |
| Terminal.app with light bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 46 04 AM"
src="https://github.com/user-attachments/assets/19bf6a3c-91e0-447b-9667-b8033f512219"
/> |
| Terminal.app with color bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 45 50 AM"
src="https://github.com/user-attachments/assets/dd7c4b5b-342e-4028-8140-f4e65752bd0b"
/> |

codex-rs/tui/src/style.rs

@@ -0,0 +1,44 @@
+use crate::color::blend;
+use crate::color::is_light;
+use crate::color::perceptual_distance;
+use crate::terminal_palette::terminal_palette;
+use ratatui::style::Color;
+use ratatui::style::Style;
+
+/// Returns the style for a user-authored message using the provided terminal background.
+pub fn user_message_style(terminal_bg: Option<(u8, u8, u8)>) -> Style {
+    match terminal_bg {
+        Some(bg) => Style::default().bg(user_message_bg(bg)),
+        None => Style::default(),
+    }
+}
+
+#[allow(clippy::disallowed_methods)]
+pub fn user_message_bg(terminal_bg: (u8, u8, u8)) -> Color {
+    let top = if is_light(terminal_bg) {
+        (0, 0, 0)
+    } else {
+        (255, 255, 255)
+    };
+    let bottom = terminal_bg;
+    let Some(color_level) = supports_color::on_cached(supports_color::Stream::Stdout) else {
+        return Color::default();
+    };
+
+    let target = blend(top, bottom, 0.1);
+    if color_level.has_16m {
+        let (r, g, b) = target;
+        Color::Rgb(r, g, b)
+    } else if color_level.has_256
+        && let Some(palette) = terminal_palette()
+        && let Some((i, _)) = palette.into_iter().enumerate().min_by(|(_, a), (_, b)| {
+            perceptual_distance(*a, target)
+                .partial_cmp(&perceptual_distance(*b, target))
+                .unwrap_or(std::cmp::Ordering::Equal)
+        })
+    {
+        Color::Indexed(i as u8)
+    } else {
+        Color::default()
+    }
+}