core/lib/Drupal/Core/Asset/CssOptimizer.php |   34 ++++++++++++++++++---------
 core/modules/color/color.module             |    3 +--
 2 files changed, 24 insertions(+), 13 deletions(-)

diff --git a/core/lib/Drupal/Core/Asset/CssOptimizer.php b/core/lib/Drupal/Core/Asset/CssOptimizer.php
index 8865623..2cf07e7 100644
--- a/core/lib/Drupal/Core/Asset/CssOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/CssOptimizer.php
@@ -14,6 +14,13 @@
 class CssOptimizer implements AssetOptimizerInterface {
 
   /**
+   * The base path used by rewriteFileURI().
+   *
+   * @var string
+   */
+  public $rewriteFileURIBasePath;
+
+  /**
    * {@inheritdoc}
    */
   public function optimize(array $css_asset) {
@@ -39,8 +46,9 @@ protected function processFile($css_asset) {
     $contents = $this->loadFile($css_asset['data'], TRUE);
 
     // Get the parent directory of this file, relative to the Drupal root.
-    $css_base_url = substr($css_asset['data'], 0, strrpos($css_asset['data'], '/'));
-    $this->rewriteFileURI(NULL, $css_base_url . '/');
+    $css_base_path = substr($css_asset['data'], 0, strrpos($css_asset['data'], '/'));
+    // Store base path.
+    $this->rewriteFileURIBasePath = $css_base_path . '/';
 
     // Anchor all paths in the CSS with its base URL, ignoring external and absolute paths.
     return preg_replace_callback('/url\(\s*[\'"]?(?![a-z]+:|\/+)([^\'")]+)[\'"]?\s*\)/i', array($this, 'rewriteFileURI'), $contents);
@@ -107,6 +115,12 @@ public function loadFile($file, $optimize = NULL, $reset_basepath = TRUE) {
    * This function is used for recursive loading of stylesheets and
    * returns the stylesheet content with all url() paths corrected.
    *
+   * @param array $matches
+   *   An array of matches by a preg_replace_callback() call that scans for
+   *   @import-ed CSS files, except for external CSS files.
+   * @return
+   *   The contents of the CSS file at $matches[1], with corrected paths.
+   *
    * @see Drupal\Core\Asset\AssetOptimizerInterface::loadFile()
    */
   protected function loadNestedFile($matches) {
@@ -191,21 +205,19 @@ protected function processCss($contents, $optimize = FALSE) {
   }
 
   /**
-   * Prefixes all paths within a CSS file for aggregateFile().
+   * Prefixes all paths within a CSS file for processFile().
+   *
+   * @param array $matches
+   *   An array of matches by a preg_replace_callback() call that scans for
+   *   url() references in CSS files, except for external or absolute ones.
    *
    * Note: the only reason this method is public is so color.module can call it;
    * it is not on the AssetOptimizerInterface, so future refactorings can make
    * it protected.
    */
-  public function rewriteFileURI($matches, $base = NULL) {
-    static $_base;
-    // Store base path for preg_replace_callback.
-    if (isset($base)) {
-      $_base = $base;
-    }
-
+  public function rewriteFileURI($matches) {
     // Prefix with base and remove '../' segments where possible.
-    $path = $_base . $matches[1];
+    $path = $this->rewriteFileURIBasePath . $matches[1];
     $last = '';
     while ($path != $last) {
       $last = $path;
diff --git a/core/modules/color/color.module b/core/modules/color/color.module
index e14a25d..7660d6b 100644
--- a/core/modules/color/color.module
+++ b/core/modules/color/color.module
@@ -388,8 +388,7 @@ function color_scheme_form_submit($form, &$form_state) {
 
       // Return the path to where this CSS file originated from, stripping
       // off the name of the file at the end of the path.
-      $base = base_path() . dirname($paths['source'] . $file) . '/';
-      $css_optimizer->rewriteFileURI(NULL, $base);
+      $css_optimizer->rewriteFileURIBasePath = base_path() . dirname($paths['source'] . $file) . '/';
 
       // Prefix all paths within this CSS file, ignoring absolute paths.
       $style = preg_replace_callback('/url\([\'"]?(?![a-z]+:|\/+)([^\'")]+)[\'"]?\)/i', array($css_optimizer, 'rewriteFileURI'), $style);