diff --git a/app/Foundation/PortableFilesystem.php b/app/Foundation/PortableFilesystem.php
new file mode 100644
index 00000000..e7e89811
--- /dev/null
+++ b/app/Foundation/PortableFilesystem.php
@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Foundation;
+
+use Hyde\Hyde;
+use Hyde\Foundation\Kernel\Filesystem;
+use Illuminate\Support\Collection;
+
+use function file_exists;
+use function in_array;
+
+/** Exposes the bundled stylesheet as a normal Portable media asset. */
+final class PortableFilesystem extends Filesystem
+{
+    public function findFiles(string $directory, string|array|false $matchExtensions = false, bool $recursive = false): Collection
+    {
+        $files = parent::findFiles($directory, $matchExtensions, $recursive);
+        $mediaDirectory = Hyde::getMediaDirectory();
+        $stylesheet = $mediaDirectory.'/app.css';
+
+        if ($directory === $mediaDirectory
+            && ! file_exists(Hyde::path($stylesheet))
+            && ! $files->contains($stylesheet)
+            && ($matchExtensions === false || in_array('css', (array) $matchExtensions, true))) {
+            $files->push($stylesheet);
+        }
+
+        return $files->sort()->values();
+    }
+}
diff --git a/app/Foundation/PortableHydeKernel.php b/app/Foundation/PortableHydeKernel.php
new file mode 100644
index 00000000..02fa0f12
--- /dev/null
+++ b/app/Foundation/PortableHydeKernel.php
@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Foundation;
+
+use Hyde\Foundation\HydeKernel;
+
+/** Hyde kernel used by the embedded application for Portable projects. */
+final class PortableHydeKernel extends HydeKernel
+{
+    public function __construct(?string $basePath = null)
+    {
+        parent::__construct($basePath);
+        $this->filesystem = new PortableFilesystem($this);
+    }
+}
diff --git a/app/Foundation/PortableIlluminateFilesystem.php b/app/Foundation/PortableIlluminateFilesystem.php
new file mode 100644
index 00000000..bcb52f08
--- /dev/null
+++ b/app/Foundation/PortableIlluminateFilesystem.php
@@ -0,0 +1,84 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Foundation;
+
+use Illuminate\Filesystem\Filesystem;
+
+use function file_get_contents;
+use function file_exists;
+use function filesize;
+use function hash_file;
+use function is_file;
+use function is_string;
+use function rtrim;
+use function str_replace;
+
+/**
+ * Adds the executable's app.css to the Portable project's read-only media view.
+ *
+ * The file is virtual: normal files always win, and no project directory is changed.
+ */
+final class PortableIlluminateFilesystem extends Filesystem
+{
+    public function __construct(
+        private readonly string $bundledStylesheet,
+        private readonly string $virtualStylesheet,
+    )
+    {
+        // Illuminate's Filesystem has no constructor; keeping this explicit makes the
+        // dependency on the bundled resource visible.
+    }
+
+    public function exists($path): bool
+    {
+        return parent::exists($path) || $this->isBundledStylesheet($path);
+    }
+
+    public function missing($path): bool
+    {
+        return ! $this->exists($path);
+    }
+
+    public function get($path, $lock = false): string
+    {
+        if (! parent::exists($path) && $this->isBundledStylesheet($path)) {
+            return (string) file_get_contents($this->bundledStylesheet);
+        }
+
+        return parent::get($path, $lock);
+    }
+
+    public function hash($path, $algorithm = 'md5')
+    {
+        if (! parent::exists($path) && $this->isBundledStylesheet($path)) {
+            return hash_file($algorithm, $this->bundledStylesheet);
+        }
+
+        return parent::hash($path, $algorithm);
+    }
+
+    public function size($path)
+    {
+        if (! parent::exists($path) && $this->isBundledStylesheet($path)) {
+            return (int) filesize($this->bundledStylesheet);
+        }
+
+        return parent::size($path);
+    }
+
+    private function isBundledStylesheet($path): bool
+    {
+        if (! is_string($path) || ! is_file($this->bundledStylesheet)) {
+            return false;
+        }
+
+        return $this->normalize($path) === $this->normalize($this->virtualStylesheet);
+    }
+
+    private function normalize(string $path): string
+    {
+        return rtrim(str_replace('\\', '/', $path), '/');
+    }
+}
diff --git a/app/Launcher/RuntimeManager.php b/app/Launcher/RuntimeManager.php
index 98d1abac..62051539 100644
--- a/app/Launcher/RuntimeManager.php
+++ b/app/Launcher/RuntimeManager.php
@@ -87,6 +87,9 @@ final class RuntimeManager
      */
     public const COMPOSER_FILE = 'composer.phar';
 
+    /** The production Hyde stylesheet used as the Portable asset fallback. */
+    public const STYLESHEET_FILE = 'app.css';
+
     /**
      * The suffix the embedded runtime binary carries inside the archive.
      *
diff --git a/app/Support/BundledStylesheet.php b/app/Support/BundledStylesheet.php
new file mode 100644
index 00000000..64709b08
--- /dev/null
+++ b/app/Support/BundledStylesheet.php
@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Support;
+
+use App\Launcher\RuntimeManager;
+use RuntimeException;
+
+use function dirname;
+use function file_exists;
+
+/** Locates the production stylesheet carried by the CLI application. */
+final class BundledStylesheet
+{
+    public static function path(): string
+    {
+        $candidates = [
+            dirname(__DIR__, 2).'/runtime/'.RuntimeManager::STYLESHEET_FILE,
+            dirname(__DIR__, 3).'/develop/packages/hyde/_media/app.css',
+            dirname(__DIR__, 3).'/develop/_media/app.css',
+        ];
+
+        foreach ($candidates as $candidate) {
+            if (file_exists($candidate)) {
+                return $candidate;
+            }
+        }
+
+        throw new RuntimeException('The bundled Hyde app.css stylesheet is missing. Rebuild the CLI or sync the Hyde develop checkout.');
+    }
+}
diff --git a/app/bootstrap.php b/app/bootstrap.php
index 01140d98..822bb96a 100644
--- a/app/bootstrap.php
+++ b/app/bootstrap.php
@@ -14,6 +14,9 @@
 
 use App\Launcher\Launcher;
 use App\Launcher\RuntimeManager;
+use App\Foundation\PortableHydeKernel;
+use App\Foundation\PortableIlluminateFilesystem;
+use App\Support\BundledStylesheet;
 
 // Deprecation notices raised by dependencies are not actionable for someone running a
 // pre-built executable, and printing them would corrupt command output. Warnings and
@@ -59,6 +62,19 @@
 
 $app = new \App\Application($environment['working']);
 
+$bundledStylesheet = $project->isPortable() ? BundledStylesheet::path() : null;
+
+if ($bundledStylesheet !== null) {
+    // Hyde's service provider applies the configured media directory while providers boot.
+    // Bind the overlay after that phase so its exact virtual path matches the kernel.
+    $app->afterBootstrapping(\Illuminate\Foundation\Bootstrap\BootProviders::class, function () use ($app, $bundledStylesheet): void {
+        $mediaDirectory = \Hyde\Hyde::getMediaDirectory();
+        $virtualStylesheet = \Hyde\Hyde::path($mediaDirectory.'/app.css');
+
+        $app->singleton(\Illuminate\Filesystem\Filesystem::class, fn (): PortableIlluminateFilesystem => new PortableIlluminateFilesystem($bundledStylesheet, $virtualStylesheet));
+    });
+}
+
 // Everything the framework writes goes outside the project being built.
 $app->useStoragePath($environment['temp'].'/app/storage');
 
@@ -123,9 +139,9 @@
 |
 */
 
-$hyde = new \Hyde\Foundation\HydeKernel(
-    $environment['working']
-);
+$hyde = $project->isPortable()
+    ? new PortableHydeKernel($environment['working'])
+    : new \Hyde\Foundation\HydeKernel($environment['working']);
 
 $app->singleton(
     \Hyde\Foundation\HydeKernel::class, function (): Hyde\Foundation\HydeKernel {
diff --git a/bin/build-phar.php b/bin/build-phar.php
index 33f45bed..d7da4581 100644
--- a/bin/build-phar.php
+++ b/bin/build-phar.php
@@ -87,6 +87,7 @@ function main(array $argv): int
 
     $version = embedRuntime($options['runtime'], $platform);
     $composer = embedComposer($options['composer'], $options['runtime']);
+    embedStylesheet();
 
     writeRuntimeManifest($version, $platform, $options['runtime'], $options['micro'], $composer);
 
@@ -108,6 +109,31 @@ function main(array $argv): int
     return verify($output, $options['micro']);
 }
 
+/** Copy the production Hyde stylesheet into the archive's runtime resources. */
+function embedStylesheet(): void
+{
+    $candidates = [
+        ROOT.'/../develop/packages/hyde/_media/app.css',
+        ROOT.'/../develop/_media/app.css',
+    ];
+
+    foreach ($candidates as $source) {
+        if (is_file($source)) {
+            $target = ROOT.'/'.RuntimeManager::RUNTIME_DIRECTORY.'/'.RuntimeManager::STYLESHEET_FILE;
+
+            if (! copy($source, $target)) {
+                fail("Unable to copy the production stylesheet from $source");
+            }
+
+            info('Stylesheet', $source);
+
+            return;
+        }
+    }
+
+    fail('Unable to find the production Hyde app.css stylesheet in the develop checkout.');
+}
+
 /**
  * Refuse to build a release artifact out of a development dependency tree.
  *
diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index 157bb05b..8e139e58 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -18,6 +18,7 @@ scattered set of file-existence checks.
 _pages/
 _posts/
 _media/
+_static/
 hyde.yml     # optional
 ```
 
@@ -29,6 +30,11 @@ never invoked at any point in a build.
 A Portable project cannot load Composer addons. There is no hybrid autoloading, and a
 local `vendor/autoload.php` is never merged into the embedded dependency graph.
 
+Portable projects remain content/configuration-only. The executable supplies the production
+Hyde stylesheet as a read-only fallback during a build when `_media/app.css` is absent; it is
+transferred to the normal compiled `media/app.css` location. `hyde new` does not create the
+source stylesheet, and a user-provided file always takes precedence.
+
 ### Composer project
 
 ```
@@ -174,12 +180,21 @@ hyde  =  micro.sfx  ++  hyde.phar
                         └── runtime/
                             ├── php.gz            (a full static PHP CLI, gzipped)
                             ├── composer.phar.gz  (Composer, patched and gzipped)
+                            ├── app.css            (the production Hyde stylesheet for Portable projects)
                             └── runtime.json      (versions, platform, checksums, patches, offset)
 ```
 
 `bin/build-native.sh` (POSIX) and `bin/build-native.ps1` (Windows) drive static-php-cli;
 `bin/build-phar.php` assembles the archive and concatenates it onto the micro binary.
 
+The production Hyde stylesheet is copied from the v3 develop checkout into `runtime/app.css`
+while the archive is assembled. In Portable mode the embedded application exposes that file
+through a read-only media overlay only when the project has no `_media/app.css`. The framework
+therefore continues to use its normal local `Asset::exists('app.css')` path: user stylesheets
+win, the source `_media/` directory is never modified, and the generated site receives the
+stylesheet at `media/app.css`. Composer projects are not given this overlay and retain their
+own asset behavior.
+
 ### Why a second PHP is embedded
 
 The micro SAPI can run the embedded application, but it is not a general CLI SAPI: it has
diff --git a/tests/Feature/NewProjectCommandTest.php b/tests/Feature/NewProjectCommandTest.php
index e3b4f5eb..ed045faa 100644
--- a/tests/Feature/NewProjectCommandTest.php
+++ b/tests/Feature/NewProjectCommandTest.php
@@ -36,6 +36,7 @@
         ->and($workspace.'/my-site/hyde.yml')->toBeFile()
         ->and($workspace.'/my-site/_posts')->toBeDirectory()
         ->and($workspace.'/my-site/_media')->toBeDirectory()
+        ->and($workspace.'/my-site/_media/app.css')->not->toBeFile()
         ->and($workspace.'/my-site/composer.json')->not->toBeFile()
         ->and($workspace.'/my-site/vendor')->not->toBeDirectory();
 });
diff --git a/tests/Feature/PortableBuildTest.php b/tests/Feature/PortableBuildTest.php
index 42bac734..c89cd5c1 100644
--- a/tests/Feature/PortableBuildTest.php
+++ b/tests/Feature/PortableBuildTest.php
@@ -27,6 +27,59 @@
         ->toContain('Test Page');
 });
 
+it('builds the bundled stylesheet locally without changing the source media directory', function () {
+    $path = TemporaryProject::portable();
+
+    $this->boot($path);
+
+    expect($this->runCommand('build'))->toBe(0)
+        ->and($path.'/_media/app.css')->not->toBeFile()
+        ->and($path.'/_site/media/app.css')->toBeFile()
+        ->and(file_get_contents($path.'/_site/index.html'))
+        ->toContain('media/app.css')
+        ->and(file_get_contents($path.'/_site/media/app.css'))
+        ->toContain('tailwindcss');
+});
+
+it('prefers and preserves a user stylesheet', function () {
+    $userStylesheet = '/* user stylesheet */\nbody { color: rebeccapurple; }\n';
+    $path = TemporaryProject::portable(['_media/app.css' => $userStylesheet]);
+
+    $this->boot($path);
+
+    expect($this->runCommand('build'))->toBe(0)
+        ->and(file_get_contents($path.'/_site/media/app.css'))->toBe($userStylesheet)
+        ->and(file_get_contents($path.'/_media/app.css'))->toBe($userStylesheet);
+});
+
+it('supports the bundled stylesheet with a custom media directory', function () {
+    $path = TemporaryProject::portable([
+        '_pages/index.md' => "# Home\n",
+        'assets/.gitkeep' => '',
+        'hyde.yml' => "name: Custom Media Site\nmedia_directory: assets\n",
+    ]);
+
+    $this->boot($path);
+
+    expect($this->runCommand('build'))->toBe(0)
+        ->and($path.'/assets/app.css')->not->toBeFile()
+        ->and($path.'/_site/assets/app.css')->toBeFile()
+        ->and(file_get_contents($path.'/_site/index.html'))
+        ->toContain('assets/app.css');
+});
+
+it('produces the same bundled stylesheet on repeated builds', function () {
+    $path = TemporaryProject::portable();
+
+    $this->boot($path);
+
+    expect($this->runCommand('build'))->toBe(0);
+    $first = file_get_contents($path.'/_site/media/app.css');
+
+    expect($this->runCommand('build'))->toBe(0)
+        ->and(file_get_contents($path.'/_site/media/app.css'))->toBe($first);
+});
+
 it('never creates a vendor directory or a composer manifest', function () {
     $path = TemporaryProject::portable();
 
diff --git a/tests/Integration/NativeExecutableTest.php b/tests/Integration/NativeExecutableTest.php
index 91adbbcc..9f193fd5 100644
--- a/tests/Integration/NativeExecutableTest.php
+++ b/tests/Integration/NativeExecutableTest.php
@@ -67,7 +67,9 @@
         ->and($path.'/_site/index.html')->toBeFile()
         ->and(file_get_contents($path.'/_site/index.html'))
         ->toContain('Hello Portable World')
-        ->toContain('Test Page');
+        ->toContain('Test Page')
+        ->toContain('media/app.css')
+        ->and($path.'/_site/media/app.css')->toBeFile();
 });
 
 it('never creates a vendor directory or a composer manifest while building', function () {