diff --git a/lib/Controller/DisplayController.php b/lib/Controller/DisplayController.php
index 645003bb..8b867169 100644
--- a/lib/Controller/DisplayController.php
+++ b/lib/Controller/DisplayController.php
@@ -26,6 +26,8 @@ class DisplayController extends Controller {
 
 	private $l10nFactory;
 
+	private $config;
+
     /**
      * @param string $AppName
      * @param IRequest $request
@@ -36,12 +38,29 @@ class DisplayController extends Controller {
 	public function __construct(string $AppName,
 								IRequest $request,
 								IURLGenerator $urlGenerator,
+								IConfig $config,
                                 L10NFactory $l10nFactory) {
 		parent::__construct($AppName, $request);
 		$this->urlGenerator = $urlGenerator;
+		$this->config = $config;
 		$this->l10nFactory = $l10nFactory;
 	}
 
+	/**
+	 * URL of the stylesheet a legacy theme provides for the editor, with its
+	 * modification time against caching, or null if the theme has none
+	 */
+	private function getThemeStylesheet(): ?string {
+		$theme = $this->config->getSystemValueString('theme', '') ?: 'default';
+		$path = '/themes/' . $theme . '/apps/files_mindmap/css/style.css';
+		// No public API exposes the server root
+		$file = \OC::$SERVERROOT . $path;
+		if (!is_file($file)) {
+			return null;
+		}
+		return $this->urlGenerator->getWebroot() . $path . '?v=' . filemtime($file);
+	}
+
 	/**
 	 * @PublicPage
 	 * @NoCSRFRequired
@@ -51,7 +70,8 @@ public function __construct(string $AppName,
 	public function showMindmapViewer(): TemplateResponse {
 		$params = [
 			'urlGenerator' => $this->urlGenerator,
-            'lang' => $this->l10nFactory->findLanguage()
+            'lang' => $this->l10nFactory->findLanguage(),
+			'themeStylesheet' => $this->getThemeStylesheet(),
 		];
 		$response = new TemplateResponse($this->appName, 'viewer', $params, 'blank');
 		$response->addHeader('Cache-Control', 'no-cache');
diff --git a/templates/viewer.php b/templates/viewer.php
index 9accc0d2..0dd52cc4 100644
--- a/templates/viewer.php
+++ b/templates/viewer.php
@@ -126,6 +126,10 @@
 			line-height: 1.2;
 		}
 	</style>
+<?php if ($themeStylesheet !== null): ?>
+	<!-- after the editor's own styles, so the theme's rules take precedence -->
+	<link rel="stylesheet" href="<?php p($themeStylesheet) ?>" />
+<?php endif; ?>
 </head>
 <script nonce="<?=$nonce?>">
     var lang = '<?=$lang?>';