diff --git a/tests/modules/symfony_mailer_test/css/inline.day.css b/tests/modules/symfony_mailer_test/css/inline.day.css
new file mode 100644
index 0000000000000000000000000000000000000000..e5034471b397a22bf63e590391ebb6d42ed94b57
--- /dev/null
+++ b/tests/modules/symfony_mailer_test/css/inline.day.css
@@ -0,0 +1,3 @@
+span.day {
+  font-style: italic;
+}
diff --git a/tests/modules/symfony_mailer_test/css/inline.text-small.css b/tests/modules/symfony_mailer_test/css/inline.text-small.css
new file mode 100644
index 0000000000000000000000000000000000000000..ef5a5489682323e2e20e94e50c1783ac5535756f
--- /dev/null
+++ b/tests/modules/symfony_mailer_test/css/inline.text-small.css
@@ -0,0 +1,5 @@
+@import url('inline.day.css');
+
+h4.text-small {
+  font-weight: bold;
+}
diff --git a/tests/modules/symfony_mailer_test/symfony_mailer_test.libraries.yml b/tests/modules/symfony_mailer_test/symfony_mailer_test.libraries.yml
new file mode 100644
index 0000000000000000000000000000000000000000..e7088048d028879f59215c454799178ba0a9368d
--- /dev/null
+++ b/tests/modules/symfony_mailer_test/symfony_mailer_test.libraries.yml
@@ -0,0 +1,4 @@
+inline_css_test:
+  css:
+    theme:
+      css/inline.text-small.css: {}
diff --git a/tests/src/Kernel/SymfonyMailerKernelTest.php b/tests/src/Kernel/SymfonyMailerKernelTest.php
index b8d4252d664ed9dcaa3667061be6899afccfe755..e6dacf27ec453010140b4fc38814ffd07fb98abb 100644
--- a/tests/src/Kernel/SymfonyMailerKernelTest.php
+++ b/tests/src/Kernel/SymfonyMailerKernelTest.php
@@ -60,4 +60,19 @@ class SymfonyMailerKernelTest extends KernelTestBase {
     $this->assertTo($to);
   }
 
+  /**
+   * Inline CSS adjuster test.
+   */
+  public function testInlineCss() {
+    $to = 'to@example.com';
+    // Test an email including the test library.
+    $this->emailFactory->newTypedEmail('symfony_mailer', 'test', $to)->addLibrary('symfony_mailer_test/inline_css_test')->send();
+    $this->readMail();
+    $this->assertNoError();
+    // The inline CSS from inline.text-small.css should appear.
+    $this->assertBodyContains('<h4 class="text-small" style="padding-top: 3px; padding-bottom: 3px; text-align: center; background-color: #0678be; color: white; font-size: smaller; font-weight: bold;">');
+    // The imported CSS from inline.day.css should appear.
+    $this->assertBodyContains('<span class="day" style="font-style: italic;">');
+  }
+
 }