diff --git a/typo3/sysext/backend/Classes/Template/DocumentTemplate.php b/typo3/sysext/backend/Classes/Template/DocumentTemplate.php
index ae2152d620d1..8088b9291c31 100644
--- a/typo3/sysext/backend/Classes/Template/DocumentTemplate.php
+++ b/typo3/sysext/backend/Classes/Template/DocumentTemplate.php
@@ -559,9 +559,8 @@ public function addStyleSheetDirectory($path)
     {
         $path = GeneralUtility::getFileAbsFileName($path);
         // Read all files in directory and sort them alphabetically
-        $cssFiles = GeneralUtility::getFilesInDir($path, 'css');
-        foreach ($cssFiles as $cssFile) {
-            $this->pageRenderer->addCssFile(PathUtility::getRelativePathTo($path) . $cssFile);
+        foreach (GeneralUtility::getFilesInDir($path, 'css', true) as $cssFile) {
+            $this->pageRenderer->addCssFile($cssFile);
         }
     }
 
diff --git a/typo3/sysext/backend/Classes/Template/ModuleTemplate.php b/typo3/sysext/backend/Classes/Template/ModuleTemplate.php
index 8134550ab9de..8dc5d8a3c592 100644
--- a/typo3/sysext/backend/Classes/Template/ModuleTemplate.php
+++ b/typo3/sysext/backend/Classes/Template/ModuleTemplate.php
@@ -294,11 +294,11 @@ protected function loadStylesheets()
         if (!empty($GLOBALS['TBE_STYLES']['stylesheet2'])) {
             $this->pageRenderer->addCssFile($GLOBALS['TBE_STYLES']['stylesheet2']);
         }
+        // @see DocumentTemplate::addStyleSheetDirectory
         // Add all *.css files of the directory $path to the stylesheets
         foreach ($this->getRegisteredStylesheetFolders() as $folder) {
             // Read all files in directory and sort them alphabetically
-            $cssFiles = GeneralUtility::getFilesInDir($folder, 'css', true);
-            foreach ($cssFiles as $cssFile) {
+            foreach (GeneralUtility::getFilesInDir($folder, 'css', true) as $cssFile) {
                 $this->pageRenderer->addCssFile($cssFile);
             }
         }
diff --git a/typo3/sysext/core/Classes/Page/PageRenderer.php b/typo3/sysext/core/Classes/Page/PageRenderer.php
index 4e1a8558db52..8661bf628eaa 100644
--- a/typo3/sysext/core/Classes/Page/PageRenderer.php
+++ b/typo3/sysext/core/Classes/Page/PageRenderer.php
@@ -16,7 +16,6 @@
 
 use TYPO3\CMS\Backend\Routing\Router;
 use TYPO3\CMS\Backend\Routing\UriBuilder;
-use TYPO3\CMS\Backend\Template\DocumentTemplate;
 use TYPO3\CMS\Core\Cache\CacheManager;
 use TYPO3\CMS\Core\Cache\Frontend\FrontendInterface;
 use TYPO3\CMS\Core\Core\Environment;
@@ -2367,12 +2366,8 @@ protected function doConcatenateCss()
                 ];
                 GeneralUtility::callUserFunction($GLOBALS['TYPO3_CONF_VARS'][TYPO3_MODE]['cssConcatenateHandler'], $params, $this);
             } else {
-                $cssOptions = [];
-                if (TYPO3_MODE === 'BE') {
-                    $cssOptions = ['baseDirectories' => GeneralUtility::makeInstance(DocumentTemplate::class)->getSkinStylesheetDirectories()];
-                }
-                $this->cssLibs = $this->getCompressor()->concatenateCssFiles($this->cssLibs, $cssOptions);
-                $this->cssFiles = $this->getCompressor()->concatenateCssFiles($this->cssFiles, $cssOptions);
+                $this->cssLibs = $this->getCompressor()->concatenateCssFiles($this->cssLibs);
+                $this->cssFiles = $this->getCompressor()->concatenateCssFiles($this->cssFiles);
             }
         }
     }
diff --git a/typo3/sysext/core/Classes/Resource/ResourceCompressor.php b/typo3/sysext/core/Classes/Resource/ResourceCompressor.php
index f16cb278cc09..2d0dae8e754f 100644
--- a/typo3/sysext/core/Classes/Resource/ResourceCompressor.php
+++ b/typo3/sysext/core/Classes/Resource/ResourceCompressor.php
@@ -103,14 +103,10 @@ public function setRootPath($rootPath)
     /**
      * Concatenates the Stylesheet files
      *
-     * Options:
-     * baseDirectories If set, only include files below one of the base directories
-     *
      * @param array $cssFiles CSS files to process
-     * @param array $options Additional options
      * @return array CSS files
      */
-    public function concatenateCssFiles(array $cssFiles, array $options = [])
+    public function concatenateCssFiles(array $cssFiles)
     {
         $filesToIncludeByType = ['all' => []];
         foreach ($cssFiles as $key => $fileOptions) {
@@ -119,45 +115,34 @@ public function concatenateCssFiles(array $cssFiles, array $options = [])
                 continue;
             }
             $filenameFromMainDir = $this->getFilenameFromMainDir($fileOptions['file']);
-            // if $options['baseDirectories'] set, we only include files below these directories
-            if (
-                !isset($options['baseDirectories'])
-                || $this->checkBaseDirectory(
-                    $filenameFromMainDir,
-                    array_merge($options['baseDirectories'], [$this->targetDirectory])
-                )
-            ) {
-                $type = isset($fileOptions['media']) ? strtolower($fileOptions['media']) : 'all';
-                if (!isset($filesToIncludeByType[$type])) {
-                    $filesToIncludeByType[$type] = [];
-                }
-                if (!empty($fileOptions['forceOnTop'])) {
-                    array_unshift($filesToIncludeByType[$type], $filenameFromMainDir);
-                } else {
-                    $filesToIncludeByType[$type][] = $filenameFromMainDir;
-                }
-                // remove the file from the incoming file array
-                unset($cssFiles[$key]);
+            $type = isset($fileOptions['media']) ? strtolower($fileOptions['media']) : 'all';
+            if (!isset($filesToIncludeByType[$type])) {
+                $filesToIncludeByType[$type] = [];
             }
+            if (!empty($fileOptions['forceOnTop'])) {
+                array_unshift($filesToIncludeByType[$type], $filenameFromMainDir);
+            } else {
+                $filesToIncludeByType[$type][] = $filenameFromMainDir;
+            }
+            // remove the file from the incoming file array
+            unset($cssFiles[$key]);
         }
-        if (!empty($filesToIncludeByType)) {
-            foreach ($filesToIncludeByType as $mediaOption => $filesToInclude) {
-                if (empty($filesToInclude)) {
-                    continue;
-                }
-                $targetFile = $this->createMergedCssFile($filesToInclude);
-                $concatenatedOptions = [
-                    'file' => $targetFile,
-                    'rel' => 'stylesheet',
-                    'media' => $mediaOption,
-                    'compress' => true,
-                    'excludeFromConcatenation' => true,
-                    'forceOnTop' => false,
-                    'allWrap' => ''
-                ];
-                // place the merged stylesheet on top of the stylesheets
-                $cssFiles = array_merge($cssFiles, [$targetFile => $concatenatedOptions]);
+        foreach ($filesToIncludeByType as $mediaOption => $filesToInclude) {
+            if (empty($filesToInclude)) {
+                continue;
             }
+            $targetFile = $this->createMergedCssFile($filesToInclude);
+            $concatenatedOptions = [
+                'file' => $targetFile,
+                'rel' => 'stylesheet',
+                'media' => $mediaOption,
+                'compress' => true,
+                'excludeFromConcatenation' => true,
+                'forceOnTop' => false,
+                'allWrap' => ''
+            ];
+            // place the merged stylesheet on top of the stylesheets
+            $cssFiles = array_merge($cssFiles, [$targetFile => $concatenatedOptions]);
         }
         return $cssFiles;
     }
@@ -447,6 +432,10 @@ protected function getFilenameFromMainDir($filename)
 
         // if the file is an absolute reference within the docRoot
         $absolutePath = $docRoot . '/' . $fileNameWithoutSlash;
+        // if it is already an absolute path to the file
+        if (PathUtility::isAbsolutePath($filename)) {
+            $absolutePath = $filename;
+        }
         // Calling is_file without @ for a path starting with '../' causes a PHP Warning when using open_basedir restriction
         if (@is_file($absolutePath)) {
             if (strpos($absolutePath, $this->rootPath) === 0) {
diff --git a/typo3/sysext/core/Documentation/Changelog/master/Breaking-88758-SelectiveConcatenationOfCSSFilesInResourceCompressorRemoved.rst b/typo3/sysext/core/Documentation/Changelog/master/Breaking-88758-SelectiveConcatenationOfCSSFilesInResourceCompressorRemoved.rst
new file mode 100644
index 000000000000..ab231a5a4e10
--- /dev/null
+++ b/typo3/sysext/core/Documentation/Changelog/master/Breaking-88758-SelectiveConcatenationOfCSSFilesInResourceCompressorRemoved.rst
@@ -0,0 +1,45 @@
+.. include:: ../../Includes.txt
+
+=====================================================================================
+Breaking: #88758 - Selective Concatenation of CSS files in ResourceCompressor removed
+=====================================================================================
+
+See :issue:`88758`
+
+Description
+===========
+
+ResourceCompressor, used to merge and compress CSS and JS files, has had an option to only
+merge CSS files from selected folders. This was used to limit CSS files of skins for TYPO3
+Backend files.
+
+The functionality has been removed, as all added CSS files are now merged into one file.
+
+As TYPO3 Frontend and TypoScript has a much more flexible system for adding CSS files,
+which should be concatenated, this change does not affect TYPO3 API of Frontend Requests.
+
+
+Impact
+======
+
+Calling ResourceCompressor->concatenateCssFiles() with a second argument has no effect anymore.
+
+Adding CSS files manually in TYPO3 Backend via custom extensions will now automatically be merged
+with the loaded CSS styles of :php:`$TBE_STYLES` skin.
+
+
+Affected Installations
+======================
+
+TYPO3 installations with extensions adding third-party CSS files in the TYPO3 Backend,
+or extensions using ResourceCompressor directly.
+
+
+Migration
+=========
+
+None, as it is considered to be useful to have one larger CSS file for TYPO3 Backend.
+
+If necessary, add a CSS file manually via PageRenderer API which should be excluded from Concatenation.
+
+.. index:: PHP-API, FullyScanned
\ No newline at end of file
diff --git a/typo3/sysext/install/Configuration/ExtensionScanner/Php/MethodArgumentDroppedMatcher.php b/typo3/sysext/install/Configuration/ExtensionScanner/Php/MethodArgumentDroppedMatcher.php
index 77aa832521ed..a690b70eab1b 100644
--- a/typo3/sysext/install/Configuration/ExtensionScanner/Php/MethodArgumentDroppedMatcher.php
+++ b/typo3/sysext/install/Configuration/ExtensionScanner/Php/MethodArgumentDroppedMatcher.php
@@ -241,4 +241,10 @@
             'Breaking-88741-CHashCalculationInIndexedSearchRemoved.rst',
         ],
     ],
+    'TYPO3\CMS\Core\Resource\ResourceCompressor->concatenateCssFiles' => [
+        'maximumNumberOfArguments' => 1,
+        'restFiles' => [
+            'Breaking-88758-SelectiveConcatenationOfCSSFilesInResourceCompressorRemoved.rst',
+        ],
+    ],
 ];