fix(border): fix flipped title colors in panel border

Revert the change to the color flipping logic for `panel` border titles.

The issue with the new method using `Style(reverse=True)` is that this
will also flip any border title colors set with Textual CSS.

Fixes #5548.

Author: TomJGooding

src/textual/_border.py

@@ -385,7 +385,10 @@ def render_border_label(
         assert False
 
     if (flip_top and is_title) or (flip_bottom and not is_title):
-        base_style = base_style.without_color + Style(reverse=True)
+        base_style = base_style.without_color + Style(
+            background=base_style.foreground,
+            foreground=base_style.background,
+        )
 
     segments = text_label.render_segments(base_style)
     yield from segments

src/textual/_styles_cache.py

@@ -379,7 +379,11 @@ def post(segments: Iterable[Segment]) -> Iterable[Segment]:
                         if label_background.a
                         else TRANSPARENT
                     ),
-                    (base_label_background + border_color + label_color),
+                    (
+                        (base_label_background + label_color)
+                        if label_color.a
+                        else TRANSPARENT
+                    ),
                 )
                 render_label = (label, style)