style: update mobile close button to green background with black icon

web/src/components/MobileReaderView.tsx

@@ -113,7 +113,7 @@ export function MobileReaderView({ article, onClose, onToggleFavorite, onNext, o
                         e.stopPropagation();
                         onClose();
                     }}
-                    className="w-10 h-10 bg-carbon/80 backdrop-blur-md text-white border border-white/10 rounded-full flex items-center justify-center active:scale-95 transition-all duration-300 pointer-events-auto hover:bg-nature shadow-lg"
+                    className="w-10 h-10 bg-nature text-carbon border border-white/10 rounded-full flex items-center justify-center active:scale-95 transition-all duration-300 pointer-events-auto hover:scale-110 shadow-lg"
                 >
                     <svg className="w-5 h-5" fill="none" viewBox="0 0 24 24" stroke="currentColor">
                         <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M6 18L18 6M6 6l12 12" />