diff --git a/lib/Horde/Themes/Css/Cache.php b/lib/Horde/Themes/Css/Cache.php
index d1f033ba..3be984fd 100644
--- a/lib/Horde/Themes/Css/Cache.php
+++ b/lib/Horde/Themes/Css/Cache.php
@@ -54,6 +54,34 @@ public function __construct(array $params = [])
      */
     abstract public function process($css, $cacheid);
 
+    /**
+     * Split off the stylesheets that are not local files (e.g. a CDN URL
+     * added by the cssfiles hook). These cannot be compressed, so they are
+     * linked as is, before the cached file. An absolute URL backed by a
+     * readable local file is kept in the cached file.
+     *
+     * @param array $css  See Horde_Themes_Css#getStylesheets().
+     *
+     * @return array  Two elements: the list of external URLs (Horde_Url
+     *                objects) and the list of local stylesheets.
+     */
+    protected function _splitExternal($css)
+    {
+        $external = $local = [];
+
+        foreach ($css as $val) {
+            if (isset($val['uri'])
+                && preg_match('#^https?://#i', (string) $val['uri'])
+                && !(isset($val['fs']) && is_readable($val['fs']))) {
+                $external[] = Horde::url($val['uri'], true, -1);
+            } else {
+                $local[] = $val;
+            }
+        }
+
+        return [$external, $local];
+    }
+
     /**
      * Perform garbage collection.
      */
diff --git a/lib/Horde/Themes/Css/Cache/File.php b/lib/Horde/Themes/Css/Cache/File.php
index 68bad5fa..e3906999 100644
--- a/lib/Horde/Themes/Css/Cache/File.php
+++ b/lib/Horde/Themes/Css/Cache/File.php
@@ -34,6 +34,11 @@ public function process($css, $cacheid)
     {
         global $registry, $injector;
 
+        [$external, $css] = $this->_splitExternal($css);
+        if (!count($css)) {
+            return $external;
+        }
+
         if (!empty($this->_params['filemtime'])) {
             foreach ($css as &$val) {
                 $val['mtime'] = @filemtime($val['fs']);
@@ -71,9 +76,9 @@ public function process($css, $cacheid)
             }
         }
 
-        return [
+        return array_merge($external, [
             Horde::url(($registry->get('staticuri', 'horde') ?? '') . '/' . $filename, true, ['append_session' => -1]),
-        ];
+        ]);
     }
 
     /**
diff --git a/lib/Horde/Themes/Css/Cache/HordeCache.php b/lib/Horde/Themes/Css/Cache/HordeCache.php
index 61eb90c9..91d04250 100644
--- a/lib/Horde/Themes/Css/Cache/HordeCache.php
+++ b/lib/Horde/Themes/Css/Cache/HordeCache.php
@@ -32,6 +32,11 @@ public function process($css, $cacheid)
     {
         global $injector;
 
+        [$external, $css] = $this->_splitExternal($css);
+        if (!count($css)) {
+            return $external;
+        }
+
         if (!empty($this->_params['filemtime'])) {
             foreach ($css as &$val) {
                 $val['mtime'] = @filemtime($val['fs']);
@@ -62,9 +67,9 @@ public function process($css, $cacheid)
             $cache->set($sig, $compress->compress($css, $logger));
         }
 
-        return [
+        return array_merge($external, [
             Horde::getCacheUrl('css', ['cid' => $sig]),
-        ];
+        ]);
     }
 
 }
diff --git a/test/Unit/Themes/Css/CacheTest.php b/test/Unit/Themes/Css/CacheTest.php
new file mode 100644
index 00000000..db323e4c
--- /dev/null
+++ b/test/Unit/Themes/Css/CacheTest.php
@@ -0,0 +1,82 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Tests for Horde_Themes_Css_Cache.
+ *
+ * @category Horde
+ * @package  Core
+ */
+
+namespace Horde\Core\Test\Unit\Themes\Css;
+
+use PHPUnit\Framework\TestCase;
+use PHPUnit\Framework\Attributes\CoversClass;
+use Horde_Registry;
+use Horde_Themes_Css_Cache;
+
+#[CoversClass(Horde_Themes_Css_Cache::class)]
+class CacheTest extends TestCase
+{
+    protected function setUp(): void
+    {
+        $GLOBALS['registry'] = $this->createStub(Horde_Registry::class);
+        $GLOBALS['registry']->method('get')->willReturn('/horde');
+    }
+
+    protected function tearDown(): void
+    {
+        unset($GLOBALS['registry']);
+    }
+
+    private function getCache(): Horde_Themes_Css_Cache
+    {
+        return new class extends Horde_Themes_Css_Cache {
+            public function process($css, $cacheid)
+            {
+                return $this->_splitExternal($css);
+            }
+        };
+    }
+
+    public function testSplitExternal(): void
+    {
+        $cdn = 'https://cdn.example.org/styles/main.css';
+        $local = ['app' => 'horde', 'fs' => '/srv/themes/x/screen.css', 'uri' => '/themes/x/screen.css'];
+
+        [$external, $rest] = $this->getCache()->process([
+            ['app' => 'horde', 'fs' => $cdn, 'uri' => $cdn],
+            $local,
+        ], '');
+
+        $this->assertCount(1, $external);
+        $this->assertSame($cdn, $external[0]->toString(false, true));
+        $this->assertSame([$local], $rest);
+    }
+
+    public function testSplitExternalKeepsAbsoluteUrlWithLocalFile(): void
+    {
+        $fs = tempnam(sys_get_temp_dir(), 'css');
+        $local = ['app' => 'horde', 'fs' => $fs, 'uri' => 'https://static.example.org/themes/x/screen.css'];
+
+        try {
+            [$external, $rest] = $this->getCache()->process([$local], '');
+        } finally {
+            unlink($fs);
+        }
+
+        $this->assertSame([], $external);
+        $this->assertSame([$local], $rest);
+    }
+
+    public function testSplitExternalWithoutExternal(): void
+    {
+        $local = ['app' => 'horde', 'fs' => '/srv/themes/x/screen.css', 'uri' => '/themes/x/screen.css'];
+
+        [$external, $rest] = $this->getCache()->process([$local], '');
+
+        $this->assertSame([], $external);
+        $this->assertSame([$local], $rest);
+    }
+}