fix(text area): fix stuck background with css theme

CHANGELOG.md

@@ -20,6 +20,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `index` to `ListView.Selected` event https://github.com/Textualize/textual/pull/5973
 - Added `layout` switch to Static.update https://github.com/Textualize/textual/pull/5973
 
+### Fixed
+
+- Fixed `TextArea` issue with the `css` theme, where the background color was stuck from the previous theme https://github.com/Textualize/textual/issues/5964
+
 ### Changed
 
 - Improved rendering of Markdown tables (replace Rich table with grid) which allows text selection https://github.com/Textualize/textual/pull/5962

src/textual/widgets/_text_area.py

@@ -860,6 +860,14 @@ def _set_theme(self, theme: str) -> None:
                     self.styles.color = Color.from_rich_color(color)
                 if background:
                     self.styles.background = Color.from_rich_color(background)
+            else:
+                # When the theme doesn't define a base style (e.g. the `css` theme),
+                # the TextArea background/color should fallback to its CSS colors.
+                #
+                # Since these styles may have already been changed by another theme,
+                # we need to reset the background/color styles to the default values.
+                self.styles.color = None
+                self.styles.background = None
 
     @property
     def available_themes(self) -> set[str]: