diff --git a/apps/theming/lib/Controller/ThemingController.php b/apps/theming/lib/Controller/ThemingController.php
index 4e9ce1b646e47..e7865b90cf048 100644
--- a/apps/theming/lib/Controller/ThemingController.php
+++ b/apps/theming/lib/Controller/ThemingController.php
@@ -51,6 +51,7 @@
 use OCA\Theming\Util;
 use OCP\ITempManager;
 use OCP\IURLGenerator;
+use OCP\App\IAppManager;
 
 /**
  * Class ThemingController
@@ -78,6 +79,8 @@ class ThemingController extends Controller {
 	private $scssCacher;
 	/** @var IURLGenerator */
 	private $urlGenerator;
+	/** @var IAppManager */
+	private $appManager;
 
 	/**
 	 * ThemingController constructor.
@@ -93,6 +96,7 @@ class ThemingController extends Controller {
 	 * @param IAppData $appData
 	 * @param SCSSCacher $scssCacher
 	 * @param IURLGenerator $urlGenerator
+	 * @param IAppManager $appManager
 	 */
 	public function __construct(
 		$appName,
@@ -105,7 +109,8 @@ public function __construct(
 		ITempManager $tempManager,
 		IAppData $appData,
 		SCSSCacher $scssCacher,
-		IURLGenerator $urlGenerator
+		IURLGenerator $urlGenerator,
+		IAppManager $appManager
 	) {
 		parent::__construct($appName, $request);
 
@@ -118,6 +123,7 @@ public function __construct(
 		$this->appData = $appData;
 		$this->scssCacher = $scssCacher;
 		$this->urlGenerator = $urlGenerator;
+		$this->appManager = $appManager;
 	}
 
 	/**
@@ -409,12 +415,13 @@ public function getLoginBackground() {
 	 * @return FileDisplayResponse|NotFoundResponse
 	 */
 	public function getStylesheet() {
-		$appPath = substr(\OC::$server->getAppManager()->getAppPath('theming'), strlen(\OC::$SERVERROOT) + 1);
+		$appPath = $this->appManager->getAppPath('theming');
+
 		/* SCSSCacher is required here
 		 * We cannot rely on automatic caching done by \OC_Util::addStyle,
 		 * since we need to add the cacheBuster value to the url
 		 */
-		$cssCached = $this->scssCacher->process(\OC::$SERVERROOT, $appPath . '/css/theming.scss', 'theming');
+		$cssCached = $this->scssCacher->process($appPath, 'css/theming.scss', 'theming');
 		if(!$cssCached) {
 			return new NotFoundResponse();
 		}
diff --git a/apps/theming/tests/Controller/ThemingControllerTest.php b/apps/theming/tests/Controller/ThemingControllerTest.php
index 54b842bc4e88c..debc1b71e4740 100644
--- a/apps/theming/tests/Controller/ThemingControllerTest.php
+++ b/apps/theming/tests/Controller/ThemingControllerTest.php
@@ -106,7 +106,8 @@ public function setUp() {
 			$this->tempManager,
 			$this->appData,
 			$this->scssCacher,
-			$this->urlGenerator
+			$this->urlGenerator,
+			$this->appManager
 		);
 
 		return parent::setUp();
@@ -798,7 +799,7 @@ public function testGetLoginBackground() {
 
 
 	public function testGetStylesheet() {
-
+		$this->appManager->expects($this->once())->method('getAppPath')->with('theming')->willReturn(\OC::$SERVERROOT . '/theming');
 		$file = $this->createMock(ISimpleFile::class);
 		$file->expects($this->any())->method('getName')->willReturn('theming.css');
 		$file->expects($this->any())->method('getContent')->willReturn('compiled');
@@ -818,6 +819,7 @@ public function testGetStylesheet() {
 	}
 
 	public function testGetStylesheetFails() {
+		$this->appManager->expects($this->once())->method('getAppPath')->with('theming')->willReturn(\OC::$SERVERROOT . '/theming');
 		$file = $this->createMock(ISimpleFile::class);
 		$file->expects($this->any())->method('getName')->willReturn('theming.css');
 		$file->expects($this->any())->method('getContent')->willReturn('compiled');
@@ -829,6 +831,26 @@ public function testGetStylesheetFails() {
 		$this->assertEquals($response, $actual);
 	}
 
+	public function testGetStylesheetOutsideServerroot() {
+		$this->appManager->expects($this->once())->method('getAppPath')->with('theming')->willReturn('/outside/serverroot/theming');
+		$file = $this->createMock(ISimpleFile::class);
+		$file->expects($this->any())->method('getName')->willReturn('theming.css');
+		$file->expects($this->any())->method('getContent')->willReturn('compiled');
+		$this->scssCacher->expects($this->once())->method('process')->with('/outside/serverroot/theming', 'css/theming.scss', 'theming')->willReturn(true);
+		$this->scssCacher->expects($this->once())->method('getCachedCSS')->willReturn($file);
+
+		$response = new Http\FileDisplayResponse($file, Http::STATUS_OK, ['Content-Type' => 'text/css']);
+		$response->cacheFor(86400);
+		$expires = new \DateTime();
+		$expires->setTimestamp($this->timeFactory->getTime());
+		$expires->add(new \DateInterval('PT24H'));
+		$response->addHeader('Expires', $expires->format(\DateTime::RFC1123));
+		$response->addHeader('Pragma', 'cache');
+
+		$actual = $this->themingController->getStylesheet();
+		$this->assertEquals($response, $actual);
+	}
+
 	public function testGetJavascript() {
 		$this->themingDefaults
 			->expects($this->at(0))