fix css never been loaded

tests/Unit/CalendarControllerTest.php

@@ -24,6 +24,8 @@ use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
 use Psr\Log\LoggerInterface;
 
+require_once __DIR__ . '/OCUtilStub.php';
+
 class CalendarControllerTest extends TestCase
 {
     private IManager&MockObject $calendarManager;
@@ -709,4 +711,20 @@ class CalendarControllerTest extends TestCase
             $response->getParams()['errors'][0]['error']
         );
     }
+
+    public function testConfirmRegistersStylesheet(): void
+    {
+        $this->userSession->method('getUser')->willReturn($this->mockUser('alice'));
+        $this->setUpWritableCalendars();
+        $this->urlGenerator->method('getAbsoluteURL')->willReturn('https://cloud.example.com/');
+
+        $c = $this->newController();
+
+        $c->confirm('Weekly sync', '', '2026-10-07T14:15', '2026-10-07T15:15');
+
+        $this->assertContains(
+            ['atocal', 'style', false],
+            \OC_Util::$styles
+        );
+    }
 }

tests/Unit/OCUtilStub.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * Minimal stand-in for the server class OC_Util, which is not part
+ * of the OCP vendor package. Records addStyle() calls for assertions.
+ */
+if (!class_exists(\OC_Util::class, false)) {
+	class OC_Util
+	{
+		/** @var list<array{string, ?string, bool}> */
+		public static array $styles = [];
+
+		public static function addStyle(
+			string $application,
+			?string $file = null,
+			bool $prepend = false
+		): void {
+			self::$styles[] = [$application, $file, $prepend];
+		}
+	}
+}