use modifier dim instead of gray and .dim (#2273)

gray color doesn't work very well with white terminals. `.dim` doesn't
have an effect for some reason.

after:
<img width="1080" height="149" alt="image"
src="https://github.com/user-attachments/assets/26c0f8bb-550d-4d71-bd06-11b3189bc1d7"
/>

Before
<img width="1077" height="186" alt="image"
src="https://github.com/user-attachments/assets/b1fba0c7-bc4d-4da1-9754-6c0a105e8cd1"
/>

codex-rs/tui/src/user_approval_widget.rs

@@ -340,7 +340,7 @@ impl WidgetRef for &UserApprovalWidget<'_> {
                 let style = if idx == self.selected_option {
                     Style::new().bg(Color::Cyan).fg(Color::Black)
                 } else {
-                    Style::new().bg(Color::DarkGray)
+                    Style::new().add_modifier(Modifier::DIM)
                 };
                 opt.label.clone().alignment(Alignment::Center).style(style)
             })
@@ -372,7 +372,7 @@ impl WidgetRef for &UserApprovalWidget<'_> {
         }
 
         Line::from(self.select_options[self.selected_option].description)
-            .style(Style::new().italic().fg(Color::DarkGray))
+            .style(Style::new().italic().add_modifier(Modifier::DIM))
             .render(description_area.inner(Margin::new(1, 0)), buf);
 
         Block::bordered()