diff --git a/src/bigocrpdf/ui/image_ocr_window.py b/src/bigocrpdf/ui/image_ocr_window.py
index e6bf042..fc71af0 100644
--- a/src/bigocrpdf/ui/image_ocr_window.py
+++ b/src/bigocrpdf/ui/image_ocr_window.py
@@ -77,6 +77,7 @@ def __init__(
             default_height=height,
         )
 
+        self.add_css_class("bigocrimage")
         self.set_title("Big Image OCR")
         self.set_icon_name("bigocrimage")
 
diff --git a/src/bigocrpdf/ui/pdf_editor/editor_window.py b/src/bigocrpdf/ui/pdf_editor/editor_window.py
index d08e127..f7852ef 100644
--- a/src/bigocrpdf/ui/pdf_editor/editor_window.py
+++ b/src/bigocrpdf/ui/pdf_editor/editor_window.py
@@ -103,6 +103,7 @@ def __init__(
             standalone: If True, show Save As button instead of Apply
         """
         super().__init__(application=application)
+        self.add_css_class("bigocrpdf-editor")
 
         self._help = EditorHelpController(self)
         self._tools = EditorToolsController(self)
diff --git a/src/bigocrpdf/utils/config_manager.py b/src/bigocrpdf/utils/config_manager.py
index 4c92021..79f3f69 100644
--- a/src/bigocrpdf/utils/config_manager.py
+++ b/src/bigocrpdf/utils/config_manager.py
@@ -40,7 +40,7 @@
 DEFAULT_CONFIG: Final[dict[str, Any]] = {
     "version": 1,
     "window": {
-        "width": 820,
+        "width": 960,  # Keep the sidebar expanded at the default text scale.
         "height": 600,
     },
     "ocr": {
diff --git a/src/bigocrpdf/window.py b/src/bigocrpdf/window.py
index eaf49b2..4bbeff2 100644
--- a/src/bigocrpdf/window.py
+++ b/src/bigocrpdf/window.py
@@ -57,7 +57,9 @@ def __init__(
             default_height=height,
         )
 
-        # The sidebar overlays the content below the 700 sp breakpoint, so the
+        self.add_css_class("bigocrpdf")
+
+        # The sidebar overlays the content in compact mode, so the
         # minimum must describe the collapsed content instead of both panes.
         self.set_size_request(640, 400)
 
diff --git a/tests/test_config_manager.py b/tests/test_config_manager.py
index 7194714..9f33a2e 100644
--- a/tests/test_config_manager.py
+++ b/tests/test_config_manager.py
@@ -13,6 +13,14 @@
 
 
 class TestConfigManager:
+    def test_default_window_width_and_saved_override(self, tmp_path):
+        manager = ConfigManager(str(tmp_path / "config.json"))
+        assert manager.get("window.width") == 960
+
+        manager.set("window.width", 820)
+        restored = ConfigManager(str(tmp_path / "config.json"))
+        assert restored.get("window.width") == 820
+
     def _make_manager(self, tmp_dir, initial=None):
         path = os.path.join(tmp_dir, "config.json")
         if initial: