[tony_sologubov]

Sorry for it, guys, this is a bug. The hotfix for it is below:

diff --git a/src/classes/XLite/Core/LessParser.php b/src/classes/XLite/Core/LessParser.php
index f29aeee..1cdeb0e 100644
--- a/src/classes/XLite/Core/LessParser.php
+++ b/src/classes/XLite/Core/LessParser.php
@@ -259,7 +259,7 @@ class LessParser extends \XLite\Base\Singleton
             $url = stripslashes(substr($url, 1, -1));
         }
 
-        $url = \Includes\Utils\FileManager::makeRelativePath($file, LC_DIR_ROOT . substr($url, strlen($rootURL)));
+        $url = str_replace(LC_DS, '/', \Includes\Utils\FileManager::makeRelativePath($file, LC_DIR_ROOT . substr($url, strlen($rootURL))));
 
         if (preg_match('/[\'"]/Ss', $url)) {
             $url = '"' . addslashes($url) . '"';
-- 

diff --git a/src/Includes/Utils/FileManager.php b/src/Includes/Utils/FileManager.php
index 707fe54..0efec9c 100644
--- a/src/Includes/Utils/FileManager.php
+++ b/src/Includes/Utils/FileManager.php
@@ -594,8 +594,6 @@ abstract class FileManager extends \Includes\Utils\AUtils
         } while ($from === $to && !empty($absFromPath) && !empty($absToPath));
 
         return str_repeat('..' . LC_DS, count($absFromPath))
-            . $to
-            . LC_DS
-            . implode(LC_DS, $absToPath);
+            . $to . (!empty($absToPath) ? (LC_DS . implode(LC_DS, $absToPath)) : '');
     }
 }
diff --git a/src/classes/XLite/Core/LessParser.php b/src/classes/XLite/Core/LessParser.php
index 1cdeb0e..97f2b28 100644
--- a/src/classes/XLite/Core/LessParser.php
+++ b/src/classes/XLite/Core/LessParser.php
@@ -261,11 +261,7 @@ class LessParser extends \XLite\Base\Singleton
 
         $url = str_replace(LC_DS, '/', \Includes\Utils\FileManager::makeRelativePath($file, LC_DIR_ROOT . substr($url, strlen($rootURL))));
 
-        if (preg_match('/[\'"]/Ss', $url)) {
-            $url = '"' . addslashes($url) . '"';
-        }
-
-        return 'url(' . $url . ')';
+        return 'url("' . $url . '")';
     }
 
     /**
---

You can safely apply it to the core files, because this fix will be already included into 5.1.5.