public function getContentCSS()
    {
        // Prioritise instance specific content
        if (isset($this->contentCSS)) {
            return $this->contentCSS;
        }

        // Add standard editor.css
        $editor = [];
        $editorCSSFiles = $this->config()->get('editor_css');
        if ($editorCSSFiles) {
            foreach ($editorCSSFiles as $editorCSS) {
                $editor[] = $editorCSS;
            }
        }

        // Themed editor.css
        $themes = HTMLEditorConfig::getThemes() ?: SSViewer::get_themes();
        $themedEditor = ThemeResourceLoader::inst()->findThemedCSS('editor', $themes);
        if ($themedEditor) {
            $editor[] = $themedEditor;
        }
        return $editor;
    }