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/history_cell.rs

@@ -1,4 +1,3 @@
-use crate::colors::LIGHT_BLUE;
 use crate::diff_render::create_diff_summary;
 use crate::exec_command::relativize_to_home;
 use crate::exec_command::strip_bash_lc_and_escape;
@@ -252,12 +251,13 @@ fn new_parsed_command(
             lines.push(Line::from(spans));
         }
         Some(o) if o.exit_code == 0 => {
-            lines.push(Line::from("✓ Completed".green().bold()));
+            lines.push(Line::from(vec!["✓".green(), " Completed".into()]));
         }
         Some(o) => {
-            lines.push(Line::from(
-                format!("✗ Failed (exit {})", o.exit_code).red().bold(),
-            ));
+            lines.push(Line::from(vec![
+                "✗".red(),
+                format!(" Failed (exit {})", o.exit_code).into(),
+            ]));
         }
     };
 
@@ -304,7 +304,7 @@ fn new_parsed_command(
             let prefix = if j == 0 { first_prefix } else { "    " };
             lines.push(Line::from(vec![
                 Span::styled(prefix, Style::default().add_modifier(Modifier::DIM)),
-                Span::styled(line_text.to_string(), Style::default().fg(LIGHT_BLUE)),
+                line_text.to_string().dim(),
             ]));
         }
     }