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/onboarding/trust_directory.rs

@@ -18,8 +18,6 @@ use ratatui::widgets::Paragraph;
 use ratatui::widgets::WidgetRef;
 use ratatui::widgets::Wrap;
 
-use crate::colors::LIGHT_BLUE;
-
 use crate::onboarding::onboarding_screen::KeyboardHandler;
 use crate::onboarding::onboarding_screen::StepStateProvider;
 
@@ -77,13 +75,7 @@ impl WidgetRef for &TrustDirectoryWidget {
             |idx: usize, option: TrustDirectorySelection, text: &str| -> Line<'static> {
                 let is_selected = self.highlighted == option;
                 if is_selected {
-                    Line::from(vec![
-                        Span::styled(
-                            format!("> {}. ", idx + 1),
-                            Style::default().fg(LIGHT_BLUE).add_modifier(Modifier::DIM),
-                        ),
-                        Span::styled(text.to_owned(), Style::default().fg(LIGHT_BLUE)),
-                    ])
+                    Line::from(format!("> {}. {text}", idx + 1)).blue()
                 } else {
                     Line::from(format!("  {}. {}", idx + 1, text))
                 }