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/styles.md

@@ -0,0 +1,22 @@
+# 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 in various terminal color themes.
+- Avoid ANSI `black` & `white` as foreground colors because the default terminal theme color 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.
+- Avoid ANSI `yellow` because for now the style guide doesn't use it. Prefer a foreground color mentioned above.
+
+(There are some rules to try to catch this in `clippy.toml`.)