[BUGFIX] Non-relative paths to the contentCSS are not accepted

Fixes: #61163

Classes/Editors/RteBase.php

@@ -32,6 +32,7 @@ use TYPO3\CMS\Backend\Utility\BackendUtility;
 use TYPO3\CMS\Core\Page\PageRenderer;
 use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;
 use TYPO3\CMS\Core\Utility\GeneralUtility;
+use TYPO3\CMS\Core\Utility\PathUtility;
 
 /**
  * A RTE using TinyMCE
@@ -90,10 +91,14 @@ class RteBase extends AbstractRte {
 			$tinyMCE = GeneralUtility::makeInstance('tinyMCE');
 			$tinyMCE->loadConfiguration($userOrPageProperties['properties']['default.']['tinymceConfiguration']);
 			if ($userOrPageProperties['properties']['default.']['contentCSS'] !== '') {
-				$tinyMCE->addConfigurationOption(
-					'content_css',
-					'/' . $userOrPageProperties['properties']['default.']['contentCSS']
-				);
+				$contentCssFile = GeneralUtility::getIndpEnv('TYPO3_SITE_URL') .
+					PathUtility::stripPathSitePrefix(
+						GeneralUtility::getFileAbsFileName(
+							$userOrPageProperties['properties']['default.']['contentCSS']
+						)
+					);
+
+				$tinyMCE->addConfigurationOption('content_css', $contentCssFile);
 			}
 			$tinyMCE->loadJsViaPageRenderer($pageRenderer, TRUE);