Add multiple contentCSS support

Classes/Form/Element/RichTextElement.php

@@ -593,15 +593,16 @@ class RichTextElement extends AbstractFormElement {
 		$tinyMCE = GeneralUtility::makeInstance(Loader::class);
 		$tinyMCE->loadConfiguration($this->vanillaRteTsConfig['properties']['default.']['tinymceConfiguration']);
 		if ($this->vanillaRteTsConfig['properties']['default.']['contentCSS'] !== '') {
-			$contentCssFile = GeneralUtility::getFileAbsFileName(
-				$this->vanillaRteTsConfig['properties']['default.']['contentCSS']
-			);
-
-			if (is_file($contentCssFile)) {
-				$contentCssFile = GeneralUtility::getIndpEnv('TYPO3_SITE_URL') .
-					PathUtility::stripPathSitePrefix($contentCssFile) . '?' . filemtime($contentCssFile);
-
-				$tinyMCE->addConfigurationOption('content_css', $contentCssFile);
+			$contentCssArray = is_array($this->vanillaRteTsConfig['properties']['default.']['contentCSS.']) ? $this->vanillaRteTsConfig['properties']['default.']['contentCSS.'] : (array)$this->vanillaRteTsConfig['properties']['default.']['contentCSS'];
+			if (!empty($contentCssArray)) {
+				$contentCssFileArray = array();
+				foreach ($contentCssArray as $contentCssKey => $contentCssFile) {
+					$contentCssFileAbs = GeneralUtility::getFileAbsFileName(trim($contentCssFile));
+					if (is_file($contentCssFileAbs)) {
+						$contentCssFileArray[] = GeneralUtility::getIndpEnv('TYPO3_SITE_URL') . PathUtility::stripPathSitePrefix($contentCssFileAbs) . '?' . filemtime($contentCssFileAbs);
+					}
+				}
+				$tinyMCE->addConfigurationOption('content_css', implode(',', $contentCssFileArray));
 			}
 		}