diff --git a/lib/private/Server.php b/lib/private/Server.php
index 5ae9cd9fdc6..28efb5c2b71 100644
--- a/lib/private/Server.php
+++ b/lib/private/Server.php
@@ -862,7 +862,7 @@ class Server extends ServerContainer implements IServerContainer {
 				$classExists = false;
 			}
 
-			if ($classExists && $c->getConfig()->getSystemValue('installed', false) && $c->getAppManager()->isInstalled('theming')) {
+			if ($classExists && $c->getConfig()->getSystemValue('installed', false) && $c->getAppManager()->isInstalled('theming') && $c->getTrustedDomainHelper()->isTrustedDomain($c->getRequest()->getInsecureServerHost())) {
 				return new ThemingDefaults(
 					$c->getConfig(),
 					$c->getL10N('theming'),