better fix

Author: willmcgugan

src/textual/app.py

@@ -1419,7 +1419,7 @@ def _validate_theme(self, theme_name: str) -> str:
             raise InvalidThemeError(message)
         return theme_name
 
-    async def _watch_theme(self, theme_name: str) -> None:
+    def _watch_theme(self, theme_name: str) -> None:
         """Apply a theme to the application.
 
         This method is called when the theme reactive attribute is set.
@@ -3657,8 +3657,9 @@ def refresh_css(self, animate: bool = True) -> None:
         stylesheet.reparse()
         stylesheet.update(self.app, animate=animate)
         try:
-            self.screen._refresh_layout(self.size)
-            self.screen._css_update_count = self._css_update_count
+            if self.screen.is_mounted:
+                self.screen._refresh_layout(self.size)
+                self.screen._css_update_count = self._css_update_count
         except ScreenError:
             pass
         # The other screens in the stack will need to know about some style