Fix CupertinoTextField incorrect background color when disabled (#78058)

packages/flutter/lib/src/cupertino/text_field.dart

@@ -1137,7 +1137,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with Restoratio
 
     final BoxDecoration? effectiveDecoration = widget.decoration?.copyWith(
       border: resolvedBorder,
-      color: enabled ? decorationColor : (decorationColor ?? disabledColor),
+      color: enabled ? decorationColor : disabledColor,
     );
 
     final Color selectionColor = CupertinoTheme.of(context).primaryColor.withOpacity(0.2);

packages/flutter/test/cupertino/text_field_test.dart

@@ -4617,4 +4617,82 @@ void main() {
       expect(state.currentTextEditingValue.composing, TextRange.empty);
     });
   });
+
+  testWidgets('disabled widget changes background color',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const CupertinoApp(
+        home: Center(
+          child: CupertinoTextField(
+            enabled: false,
+          ),
+        ),
+      ),
+    );
+
+    BoxDecoration decoration = tester
+        .widget<DecoratedBox>(
+          find.descendant(
+            of: find.byType(CupertinoTextField),
+            matching: find.byType(DecoratedBox),
+          ),
+        )
+        .decoration as BoxDecoration;
+
+    expect(
+      decoration.color!.value,
+      0xFFFAFAFA,
+    );
+
+    await tester.pumpWidget(
+      const CupertinoApp(
+        home: Center(
+          child: CupertinoTextField(
+            enabled: true,
+          ),
+        ),
+      ),
+    );
+
+    decoration = tester
+        .widget<DecoratedBox>(
+          find.descendant(
+            of: find.byType(CupertinoTextField),
+            matching: find.byType(DecoratedBox),
+          ),
+        )
+        .decoration as BoxDecoration;
+
+    expect(
+      decoration.color!.value,
+      CupertinoColors.white.value,
+    );
+
+    await tester.pumpWidget(
+      const CupertinoApp(
+        theme: CupertinoThemeData(
+          brightness: Brightness.dark,
+        ),
+        home: Center(
+          child: CupertinoTextField(
+            enabled: false,
+          ),
+        ),
+      ),
+    );
+
+    decoration = tester
+        .widget<DecoratedBox>(
+          find.descendant(
+            of: find.byType(CupertinoTextField),
+            matching: find.byType(DecoratedBox),
+          ),
+        )
+        .decoration as BoxDecoration;
+
+    expect(
+      decoration.color!.value,
+      0xFF050505,
+    );
+  });
 }