Refactor dark mode color blending to extension method

Moved dark mode color blending logic to a Color extension (toDark) in the design system. Updated usages in ui_utils.dart and related tests to use the new extension. Removed the old getDarkModeColor function for cleaner and more idiomatic code organization.

test/widgets/texts_test.dart

@@ -49,7 +49,7 @@ void main() {
     expect(find.byType(SizedBox), findsOneWidget);
     expect(find.text('DEL'), findsOneWidget);
     expect(find.text('GET'), findsNothing);
-    Color colDelDarkMode = getDarkModeColor(kColorHttpMethodDelete);
+    Color colDelDarkMode = kColorHttpMethodDelete.toDark;
     final delTextWithColor = find.byWidgetPredicate(
         (widget) => widget is Text && widget.style!.color == colDelDarkMode);
     expect(delTextWithColor, findsOneWidget);