test: add a testcase

Author: shuoweil

tests/unit/display/test_anywidget.py

@@ -96,3 +96,27 @@ def test_css_contains_dark_mode_media_query():
     with mock.patch.object(TableWidget, "_initial_load"):
         widget = TableWidget(mock_df)
         assert "@media (prefers-color-scheme: dark)" in widget._css
+
+
+def test_css_styles_traitlet_is_populated():
+    """Verify that css_styles traitlet is populated from the external CSS file."""
+    from bigframes.display.anywidget import TableWidget
+
+    # Mock the dataframe and its block
+    mock_df = mock.create_autospec(bigframes.dataframe.DataFrame, instance=True)
+    mock_df.columns = ["col1"]
+    mock_df.dtypes = {"col1": "object"}
+    mock_block = mock.Mock()
+    mock_block.has_index = False
+    mock_df._block = mock_block
+
+    # Mock _initial_load to avoid side effects
+    with mock.patch.object(TableWidget, "_initial_load"):
+        widget = TableWidget(mock_df)
+
+        # Check that css_styles is not empty
+        assert widget.css_styles
+
+        # Check that it contains expected CSS content (e.g. dark mode query)
+        assert "@media (prefers-color-scheme: dark)" in widget.css_styles
+        assert ".bigframes-widget" in widget.css_styles