fix: clean up styles & colors and define in styles.md (#2401)

New style guide:

  # Headers, primary, and secondary text
  
- **Headers:** Use `bold`. For markdown with various header levels,
leave in the `#` signs.
  - **Primary text:** Default.
  - **Secondary text:** Use `dim`.
  
  # Foreground colors
  
- **Default:** Most of the time, just use the default foreground color.
`reset` can help get it back.
- **Selection:** Use ANSI `blue`. (Ed & AE want to make this cyan too,
but we'll do that in a followup since it's riskier in different themes.)
  - **User input tips and status indicators:** Use ANSI `cyan`.
  - **Success and additions:** Use ANSI `green`.
  - **Errors, failures and deletions:** Use ANSI `red`.
  - **Codex:** Use ANSI `magenta`.
  
  # Avoid
  
- Avoid custom colors because there's no guarantee that they'll contrast
well or look good on various terminal color themes.
- Avoid ANSI `black`, `white`, `yellow` as foreground colors because the
terminal theme will do a better job. (Use `reset` if you need to in
order to get those.) The exception is if you need contrast rendering
over a manually colored background.
  
  (There are some rules to try to catch this in `clippy.toml`.)

# Testing

Tested in a variety of light and dark color themes in Terminal, iTerm2, and Ghostty.

codex-rs/tui/src/shimmer.rs

@@ -46,9 +46,14 @@ pub(crate) fn shimmer_spans(text: &str) -> Vec<Span<'static>> {
         let brightness = 0.4 + 0.6 * t;
         let level = (brightness * 255.0).clamp(0.0, 255.0) as u8;
         let style = if has_true_color {
-            Style::default()
-                .fg(Color::Rgb(level, level, level))
-                .add_modifier(Modifier::BOLD)
+            // Allow custom RGB colors, as the implementation is thoughtfully
+            // adjusting the level of the default foreground color.
+            #[allow(clippy::disallowed_methods)]
+            {
+                Style::default()
+                    .fg(Color::Rgb(level, level, level))
+                    .add_modifier(Modifier::BOLD)
+            }
         } else {
             color_for_level(level)
         };