diff --git a/.horde.yml b/.horde.yml
index 5f4d28bb..c0c95a16 100644
--- a/.horde.yml
+++ b/.horde.yml
@@ -55,7 +55,7 @@ dependencies:
       horde/compress: ^3
       horde/compress_fast: ^2
       horde/controller: ^3
-      horde/cssminify: ^2
+      horde/cssminify: ^2.0.0-beta2
       horde/data: ^3
       horde/date: ^3
       horde/exception: ^3
diff --git a/lib/Horde/Themes/Css.php b/lib/Horde/Themes/Css.php
index ddc9c563..e0d9aa4a 100644
--- a/lib/Horde/Themes/Css.php
+++ b/lib/Horde/Themes/Css.php
@@ -14,6 +14,9 @@
  * @license  http://www.horde.org/licenses/lgpl21 LGPL 2.1
  * @package  Core
  */
+
+use Horde\Log\Logger;
+
 class Horde_Themes_Css
 {
     /**
@@ -253,8 +256,19 @@ public function getBaseStylesheetList()
      */
     public function loadCssFiles($files)
     {
+        global $injector;
+
         $compress = new Horde_Themes_Css_Compress();
-        return $compress->compress($files);
+
+        // Try to get PSR-3 logger for error reporting
+        $logger = null;
+        try {
+            $logger = $injector->get(Logger::class);
+        } catch (Exception $e) {
+            // Logger not available, continue without logging
+        }
+
+        return $compress->compress($files, $logger);
     }
 
 }
diff --git a/lib/Horde/Themes/Css/Cache/File.php b/lib/Horde/Themes/Css/Cache/File.php
index 03e96d41..cbcac422 100644
--- a/lib/Horde/Themes/Css/Cache/File.php
+++ b/lib/Horde/Themes/Css/Cache/File.php
@@ -12,6 +12,8 @@
  * @package   Core
  */
 
+use Horde\Log\Logger;
+
 /**
  * Filesystem backend for the CSS caching library.
  *
@@ -28,7 +30,7 @@ class Horde_Themes_Css_Cache_File extends Horde_Themes_Css_Cache
      */
     public function process($css, $cacheid)
     {
-        global $registry;
+        global $registry, $injector;
 
         if (!empty($this->_params['filemtime'])) {
             foreach ($css as &$val) {
@@ -49,8 +51,17 @@ public function process($css, $cacheid)
 
         if (!file_exists($path)) {
             $compress = new Horde_Themes_Css_Compress();
+
+            // Try to get PSR-3 logger for error reporting
+            $logger = null;
+            try {
+                $logger = $injector->get(Logger::class);
+            } catch (Exception $e) {
+                // Logger not available, continue without logging
+            }
+
             $temp = Horde_Util::getTempFile('staticcss', true, $js_fs);
-            if (!file_put_contents($temp, $compress->compress($css), LOCK_EX) ||
+            if (!file_put_contents($temp, $compress->compress($css, $logger), LOCK_EX) ||
                 !chmod($temp, 0o777 & ~umask()) ||
                 !rename($temp, $path)) {
                 Horde::log('Could not write cached CSS file to disk.', 'EMERG');
diff --git a/lib/Horde/Themes/Css/Cache/HordeCache.php b/lib/Horde/Themes/Css/Cache/HordeCache.php
index 920535c2..3c832ccd 100644
--- a/lib/Horde/Themes/Css/Cache/HordeCache.php
+++ b/lib/Horde/Themes/Css/Cache/HordeCache.php
@@ -12,6 +12,8 @@
  * @package   Core
  */
 
+use Horde\Log\Logger;
+
 /**
  * Horde_Cache backend for the CSS caching library.
  *
@@ -48,7 +50,16 @@ public function process($css, $cacheid)
         // Do lifetime checking here, not on cache display page.
         if (!$cache->exists($sig, empty($this->_params['lifetime']) ? 0 : $this->_params['lifetime'])) {
             $compress = new Horde_Themes_Css_Compress();
-            $cache->set($sig, $compress->compress($css));
+
+            // Try to get PSR-3 logger for error reporting
+            $logger = null;
+            try {
+                $logger = $injector->get(Logger::class);
+            } catch (Exception $e) {
+                // Logger not available, continue without logging
+            }
+
+            $cache->set($sig, $compress->compress($css, $logger));
         }
 
         return [
diff --git a/lib/Horde/Themes/Css/Compress.php b/lib/Horde/Themes/Css/Compress.php
index 08328f23..6b81a94a 100644
--- a/lib/Horde/Themes/Css/Compress.php
+++ b/lib/Horde/Themes/Css/Compress.php
@@ -1,23 +1,32 @@
 <?php
 
 /**
- * Copyright 2014-2017 Horde LLC (http://www.horde.org/)
+ * Copyright 2014-2026 Horde LLC (http://www.horde.org/)
  *
  * See the enclosed file LICENSE for license information (LGPL). If you
  * did not receive this file, see http://www.horde.org/licenses/lgpl21.
  *
  * @category  Horde
- * @copyright 2014-2017 Horde LLC
+ * @copyright 2014-2026 Horde LLC
  * @license   http://www.horde.org/licenses/lgpl21 LGPL 2.1
  * @package   Core
  */
 
+use Horde\CssMinify\CssParserMinifier;
+use Horde\CssMinify\ImportCallback;
+use Horde\CssMinify\Input\CssFile;
+use Horde\CssMinify\Input\FileCollectionInput;
+use Horde\CssMinify\Settings;
+use Horde\CssMinify\UrlCallback;
+use Horde\Log\Logger;
+use Psr\Log\LoggerInterface;
+
 /**
  * Compresses CSS based on Horde configuration parameters.
  *
  * @author    Michael Slusarz <slusarz@horde.org>
  * @category  Horde
- * @copyright 2014-2017 Horde LLC
+ * @copyright 2014-2026 Horde LLC
  * @license   http://www.horde.org/licenses/lgpl21 LGPL 2.1
  * @package   Core
  * @since     2.12.0
@@ -29,25 +38,62 @@ class Horde_Themes_Css_Compress
      * to a string.
      *
      * @param array $css  See Horde_Themes_Css#getStylesheets().
+     * @param LoggerInterface|null $logger  Optional PSR-3 logger for error reporting.
      *
      * @return string  CSS data.
      */
-    public function compress($css)
+    public function compress($css, ?LoggerInterface $logger = null)
     {
         global $browser, $conf, $injector;
 
         $files = [];
         foreach ($css as $val) {
-            $files[$val['uri']] = $val['fs'];
+            if (!isset($val['uri']) || !isset($val['fs'])) {
+                continue;
+            }
+            try {
+                $files[] = new CssFile((string) $val['uri'], (string) $val['fs']);
+            } catch (InvalidArgumentException $e) {
+                // Skip unreadable files
+                if ($logger !== null) {
+                    $logger->warning(
+                        'Skipping unreadable CSS file: {file}',
+                        ['file' => $val['fs'] ?? 'unknown', 'exception' => $e->getMessage()]
+                    );
+                }
+                continue;
+            }
+        }
+
+        if (empty($files)) {
+            return '';
+        }
+
+        $dataUrlCallback = null;
+        if (empty($conf['nobase64_img']) && $browser->hasFeature('dataurl')) {
+            $dataUrlCallback = new UrlCallback([$this, 'dataurlCallback']);
+        }
+
+        // Use provided logger or attempt to get from injector
+        if ($logger === null) {
+            try {
+                $logger = $injector->get(Logger::class);
+            } catch (Exception $e) {
+                // Logger not available, continue without logging
+                $logger = null;
+            }
         }
 
-        $parser = new Horde_CssMinify_CssParser($files, [
-            'dataurl' => (empty($conf['nobase64_img']) && $browser->hasFeature('dataurl')) ? [$this, 'dataurlCallback'] : null,
-            'import' => [$this, 'importCallback'],
-            'logger' => $injector->getInstance('Horde_Log_Logger'),
-        ]);
+        $minifier = new CssParserMinifier(
+            new FileCollectionInput(...$files),
+            new Settings(
+                dataUrlCallback: $dataUrlCallback,
+                importCallback: new ImportCallback([$this, 'importCallback']),
+                logger: $logger
+            )
+        );
 
-        return $parser->minify();
+        return $minifier->minify();
     }
 
     /**
diff --git a/test/Horde_Themes_Css_CompressTest.php b/test/Horde_Themes_Css_CompressTest.php
new file mode 100644
index 00000000..b979383d
--- /dev/null
+++ b/test/Horde_Themes_Css_CompressTest.php
@@ -0,0 +1,145 @@
+<?php
+
+/**
+ * Tests for Horde_Themes_Css_Compress using modern CssMinify API.
+ *
+ * @category Horde
+ * @package  Core
+ */
+class Horde_Themes_Css_CompressTest extends PHPUnit\Framework\TestCase
+{
+    private string $fixturesPath;
+
+    protected function setUp(): void
+    {
+        $this->fixturesPath = __DIR__ . '/fixtures/';
+
+        // Create fixtures directory if it doesn't exist
+        if (!is_dir($this->fixturesPath)) {
+            mkdir($this->fixturesPath, 0o777, true);
+        }
+
+        // Create test CSS file
+        file_put_contents(
+            $this->fixturesPath . 'test.css',
+            'body { color: red; margin: 10px; }'
+        );
+
+        file_put_contents(
+            $this->fixturesPath . 'with-import.css',
+            '@import "shared.css"; body { background: white; }'
+        );
+
+        file_put_contents(
+            $this->fixturesPath . 'shared.css',
+            '.header { padding: 20px; }'
+        );
+
+        file_put_contents(
+            $this->fixturesPath . 'with-url.css',
+            'body { background-image: url(../images/bg.png); }'
+        );
+    }
+
+    protected function tearDown(): void
+    {
+        // Cleanup fixtures
+        $files = glob($this->fixturesPath . '*.css');
+        foreach ($files as $file) {
+            unlink($file);
+        }
+        if (is_dir($this->fixturesPath)) {
+            rmdir($this->fixturesPath);
+        }
+    }
+
+    public function testCompressBasicCss(): void
+    {
+        $compress = new Horde_Themes_Css_Compress();
+
+        $css = [
+            ['uri' => 'test.css', 'fs' => $this->fixturesPath . 'test.css'],
+        ];
+
+        $result = $compress->compress($css);
+
+        $this->assertStringContainsString('color:red', $result);
+        $this->assertStringContainsString('margin:10px', $result);
+    }
+
+    public function testCompressMultipleFiles(): void
+    {
+        $compress = new Horde_Themes_Css_Compress();
+
+        $css = [
+            ['uri' => 'test.css', 'fs' => $this->fixturesPath . 'test.css'],
+            ['uri' => 'shared.css', 'fs' => $this->fixturesPath . 'shared.css'],
+        ];
+
+        $result = $compress->compress($css);
+
+        $this->assertStringContainsString('color:red', $result);
+        $this->assertStringContainsString('.header', $result);
+    }
+
+    public function testCompressHandlesInvalidFiles(): void
+    {
+        $compress = new Horde_Themes_Css_Compress();
+
+        $css = [
+            ['uri' => 'missing.css', 'fs' => $this->fixturesPath . 'nonexistent.css'],
+        ];
+
+        // Should not throw, returns empty string
+        $result = $compress->compress($css);
+
+        $this->assertSame('', $result);
+    }
+
+    public function testCompressHandlesMissingKeys(): void
+    {
+        $compress = new Horde_Themes_Css_Compress();
+
+        $css = [
+            ['uri' => 'test.css'], // Missing 'fs'
+            ['fs' => $this->fixturesPath . 'test.css'], // Missing 'uri'
+            [], // Missing both
+        ];
+
+        // Should not throw, skips invalid entries
+        $result = $compress->compress($css);
+
+        $this->assertSame('', $result);
+    }
+
+    public function testCompressHandlesEmptyInput(): void
+    {
+        $compress = new Horde_Themes_Css_Compress();
+
+        $result = $compress->compress([]);
+
+        $this->assertSame('', $result);
+    }
+
+    public function testDataurlCallback(): void
+    {
+        $compress = new Horde_Themes_Css_Compress();
+
+        // Test callback is callable
+        $result = $compress->dataurlCallback('/path/to/image.png');
+
+        // Should return something (actual implementation depends on Horde_Themes_Image)
+        $this->assertIsString($result);
+    }
+
+    public function testImportCallback(): void
+    {
+        $compress = new Horde_Themes_Css_Compress();
+
+        // Test callback returns array with uri and fs
+        $result = $compress->importCallback('test.css');
+
+        $this->assertIsArray($result);
+        $this->assertCount(2, $result);
+    }
+}
diff --git a/test/Horde_Themes_Css_Compress_IntegrationTest.php b/test/Horde_Themes_Css_Compress_IntegrationTest.php
new file mode 100644
index 00000000..09dacf10
--- /dev/null
+++ b/test/Horde_Themes_Css_Compress_IntegrationTest.php
@@ -0,0 +1,90 @@
+<?php
+
+/**
+ * Integration test for Horde_Themes_Css_Compress with modern CssMinify.
+ *
+ * This test verifies that the updated Compress class can instantiate
+ * the modern CssMinify API correctly (CssFile validation, type safety).
+ *
+ * @category Horde
+ * @package  Core
+ */
+class Horde_Themes_Css_Compress_IntegrationTest extends PHPUnit\Framework\TestCase
+{
+    public function testModernApiCanBeInstantiated(): void
+    {
+        // Verify modern CssMinify classes exist
+        $this->assertTrue(class_exists('Horde\CssMinify\CssParserMinifier'));
+        $this->assertTrue(class_exists('Horde\CssMinify\Input\CssFile'));
+        $this->assertTrue(class_exists('Horde\CssMinify\Input\FileCollectionInput'));
+        $this->assertTrue(class_exists('Horde\CssMinify\Settings'));
+        $this->assertTrue(class_exists('Horde\CssMinify\UrlCallback'));
+        $this->assertTrue(class_exists('Horde\CssMinify\ImportCallback'));
+    }
+
+    public function testCssFileValidation(): void
+    {
+        $tempFile = tempnam(sys_get_temp_dir(), 'css');
+        file_put_contents($tempFile, 'body { color: red; }');
+
+        // Modern API validates at construction
+        $file = new \Horde\CssMinify\Input\CssFile('test.css', $tempFile);
+
+        $this->assertSame('test.css', $file->uri);
+        $this->assertSame($tempFile, $file->filepath);
+
+        unlink($tempFile);
+    }
+
+    public function testCssFileThrowsOnInvalid(): void
+    {
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('File not readable');
+
+        new \Horde\CssMinify\Input\CssFile('missing.css', '/nonexistent/path.css');
+    }
+
+    public function testModernApiTypeSafety(): void
+    {
+        $tempFile = tempnam(sys_get_temp_dir(), 'css');
+        file_put_contents($tempFile, 'body { color: red; }');
+
+        $file = new \Horde\CssMinify\Input\CssFile('test.css', $tempFile);
+        $collection = new \Horde\CssMinify\Input\FileCollectionInput($file);
+        $minifier = new \Horde\CssMinify\CssParserMinifier($collection);
+
+        $result = $minifier->minify();
+
+        $this->assertIsString($result);
+        $this->assertStringContainsString('color:red', $result);
+
+        unlink($tempFile);
+    }
+
+    public function testSettingsImmutability(): void
+    {
+        $urlCallback = new \Horde\CssMinify\UrlCallback(fn ($p) => $p);
+        $settings = new \Horde\CssMinify\Settings(dataUrlCallback: $urlCallback);
+
+        // Properties are readonly
+        $this->assertSame($urlCallback, $settings->dataUrlCallback);
+    }
+
+    public function testCompressCodePath(): void
+    {
+        // This verifies the updated Compress.php code paths compile
+        $reflection = new \ReflectionClass('Horde_Themes_Css_Compress');
+
+        // Verify compress method exists
+        $this->assertTrue($reflection->hasMethod('compress'));
+
+        // Verify it uses modern API classes (check use statements in file)
+        $filename = $reflection->getFileName();
+        $content = file_get_contents($filename);
+
+        $this->assertStringContainsString('use Horde\CssMinify\CssParserMinifier', $content);
+        $this->assertStringContainsString('use Horde\CssMinify\Input\CssFile', $content);
+        $this->assertStringContainsString('use Horde\CssMinify\Input\FileCollectionInput', $content);
+        $this->assertStringContainsString('use Horde\CssMinify\Settings', $content);
+    }
+}