fix for setting the theme early

CHANGELOG.md

@@ -31,6 +31,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Fixed issue where Segments with a style of `None` aren't rendered https://github.com/Textualize/textual/pull/6109
 - Fixed visual glitches and crash when changing `DataTable.header_height` https://github.com/Textualize/textual/pull/6128
 - Fixed TextArea.placeholder not handling multi-lines https://github.com/Textualize/textual/pull/6138
+- Fixed issue with RichLog when App.theme is set early https://github.com/Textualize/textual/pull/6141
 
 ## [6.1.0] - 2025-08-01
 

src/textual/app.py

@@ -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

tests/snapshot_tests/test_snapshots.py

@@ -4654,3 +4654,25 @@ def compose(self) -> ComposeResult:
             yield TextArea(placeholder=TEXT)
 
     assert snap_compare(PlaceholderApp())
+
+
+def test_rich_log_early_write(snap_compare) -> None:
+    """Regression test for https://github.com/Textualize/textual/issues/6123
+
+    You should see a RichLog with "Hello World" text
+
+    """
+
+    class TestApp(App):
+        def compose(self) -> ComposeResult:
+            with Horizontal():
+                yield RichLog()
+
+        def on_mount(self) -> None:
+            self.theme = "nord"
+
+        def on_ready(self) -> None:
+            log_widget = self.query_one(RichLog)
+            log_widget.write("Hello, World!")
+
+    assert snap_compare(TestApp())