Fix outline-top and outline-bottom not handling colour alpha

See #2371.

CHANGELOG.md

@@ -5,6 +5,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## Unreleased
+
+### Fixed
+
+- Fixed `outline` top and bottom not handling alpha - https://github.com/Textualize/textual/issues/2371
 
 ## [0.22.3] - 2023-04-29
 

src/textual/_styles_cache.py

@@ -432,7 +432,7 @@ class StylesCache:
                 outline_top if y == 0 else outline_bottom,
                 inner,
                 outer,
-                from_color(color=outline_color.rich_color),
+                from_color(color=(base_background + outline_color).rich_color),
             )
             line = render_row(
                 box_segments[0 if y == 0 else 2],