TUI: change the diff preview to have color fg not bg (#2270)

<img width="328" height="95" alt="image"
src="https://github.com/user-attachments/assets/70e1e6c2-a88f-4058-8763-85c3a02eedb4"
/>

Author: aibrahim-oai

codex-rs/tui/src/diff_render.rs

@@ -270,13 +270,14 @@ fn push_wrapped_diff_line(
 
     // Reserve a fixed number of spaces after the line number so that content starts
     // at a consistent column. The sign ("+"/"-") is rendered as part of the content
-    // with the same background as the edit, not as a separate dimmed column.
+    // and colored with the same foreground as the edited text, not as a separate
+    // dimmed column.
     let gap_after_ln = SPACES_AFTER_LINE_NUMBER.saturating_sub(ln_str.len());
     let first_prefix_cols = indent.len() + ln_str.len() + gap_after_ln;
     let cont_prefix_cols = indent.len() + ln_str.len() + gap_after_ln;
 
     let mut first = true;
-    let (sign_opt, bg_style) = match kind {
+    let (sign_opt, line_style) = match kind {
         DiffLineType::Insert => (Some('+'), Some(style_add())),
         DiffLineType::Delete => (Some('-'), Some(style_del())),
         DiffLineType::Context => (None, None),
@@ -307,30 +308,38 @@ fn push_wrapped_diff_line(
             spans.push(RtSpan::raw(" ".repeat(gap_after_ln)));
 
             // Prefix the content with the sign if it is an insertion or deletion, and color
-            // the sign with the same background as the edited text.
+            // the sign and content with the same foreground as the edited text.
             let display_chunk = match sign_opt {
                 Some(sign_char) => format!("{sign_char}{chunk}"),
                 None => chunk.to_string(),
             };
 
-            let content_span = match bg_style {
+            let content_span = match line_style {
                 Some(style) => RtSpan::styled(display_chunk, style),
                 None => RtSpan::raw(display_chunk),
             };
             spans.push(content_span);
-            lines.push(RtLine::from(spans));
+            let mut line = RtLine::from(spans);
+            if let Some(style) = line_style {
+                line.style = line.style.patch(style);
+            }
+            lines.push(line);
             first = false;
         } else {
             let hang_prefix = format!(
                 "{indent}{}{}",
                 " ".repeat(ln_str.len()),
                 " ".repeat(gap_after_ln)
             );
-            let content_span = match bg_style {
+            let content_span = match line_style {
                 Some(style) => RtSpan::styled(chunk.to_string(), style),
                 None => RtSpan::raw(chunk.to_string()),
             };
-            lines.push(RtLine::from(vec![RtSpan::raw(hang_prefix), content_span]));
+            let mut line = RtLine::from(vec![RtSpan::raw(hang_prefix), content_span]);
+            if let Some(style) = line_style {
+                line.style = line.style.patch(style);
+            }
+            lines.push(line);
         }
     }
     lines
@@ -341,11 +350,11 @@ fn style_dim() -> Style {
 }
 
 fn style_add() -> Style {
-    Style::default().bg(Color::Green)
+    Style::default().fg(Color::Green)
 }
 
 fn style_del() -> Style {
-    Style::default().bg(Color::Red)
+    Style::default().fg(Color::Red)
 }
 
 #[allow(clippy::expect_used)]