diff --git a/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileExistingSettingsTest.php b/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileExistingSettingsTest.php
index 7e19154118cedff2716f06cc31583cf95b0cae02..e8f77bd945c91c226825e5b0687b381225209878 100644
--- a/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileExistingSettingsTest.php
+++ b/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileExistingSettingsTest.php
@@ -38,7 +38,7 @@ protected function prepareEnvironment() {
       'distribution' => [
         'name' => 'My Distribution',
         'install' => [
-          'theme' => 'bartik',
+          'theme' => 'olivero',
         ],
       ],
     ];
diff --git a/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTest.php b/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTest.php
index d8efca525c1f0d5b89917ac8878bfae41a6c247f..68a9cd1485c57b6eb45145dd57d3c7bfbd03cf17 100644
--- a/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTest.php
+++ b/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTest.php
@@ -32,7 +32,7 @@ protected function prepareEnvironment() {
       'distribution' => [
         'name' => 'My Distribution',
         'install' => [
-          'theme' => 'bartik',
+          'theme' => 'claro',
           'finish_url' => '/root-user',
         ],
       ],
diff --git a/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTranslationQueryTest.php b/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTranslationQueryTest.php
index a83452e3399b002104d9a80afa50d4054f5c118b..2f7e08a9209206076a4f443cf14b9b8ab89b94d4 100644
--- a/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTranslationQueryTest.php
+++ b/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTranslationQueryTest.php
@@ -43,7 +43,7 @@ protected function prepareEnvironment() {
         'name' => 'My Distribution',
         'langcode' => $this->langcode,
         'install' => [
-          'theme' => 'bartik',
+          'theme' => 'claro',
         ],
       ],
     ];
diff --git a/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTranslationTest.php b/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTranslationTest.php
index 90c8c4c62c519f50021b93cbdcbb72d09bea29ab..abf93bd65e84a585c6dd241269c0a6e3b1d0f4e5 100644
--- a/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTranslationTest.php
+++ b/core/tests/Drupal/FunctionalTests/Installer/DistributionProfileTranslationTest.php
@@ -45,7 +45,7 @@ protected function prepareEnvironment() {
         'name' => 'My Distribution',
         'langcode' => $this->langcode,
         'install' => [
-          'theme' => 'bartik',
+          'theme' => 'claro',
         ],
       ],
     ];
diff --git a/core/tests/Drupal/FunctionalTests/Installer/InstallerTest.php b/core/tests/Drupal/FunctionalTests/Installer/InstallerTest.php
index 4d1df63869e4b7b507cf2f007a833b51ddffae25..12a2f0ddcf163bbe737a6cb60cf4a5f0c10ec751 100644
--- a/core/tests/Drupal/FunctionalTests/Installer/InstallerTest.php
+++ b/core/tests/Drupal/FunctionalTests/Installer/InstallerTest.php
@@ -53,7 +53,7 @@ public function testInstaller() {
   protected function setUpLanguage() {
     // Test that \Drupal\Core\Render\BareHtmlPageRenderer adds assets and
     // metatags as expected to the first page of the installer.
-    $this->assertSession()->responseContains("core/themes/claro/css/components/button.css");
+    $this->assertSession()->responseContains("css/components/button.css");
     $this->assertSession()->responseContains('<meta charset="utf-8" />');
 
     // Assert that the expected title is present.
diff --git a/core/tests/Drupal/FunctionalTests/Installer/MultipleDistributionsProfileTest.php b/core/tests/Drupal/FunctionalTests/Installer/MultipleDistributionsProfileTest.php
index ca71d7fb04270d053b65c47edab5563a93be044d..b7754682db491aa8b5a7547ec5582c61b9b48f5d 100644
--- a/core/tests/Drupal/FunctionalTests/Installer/MultipleDistributionsProfileTest.php
+++ b/core/tests/Drupal/FunctionalTests/Installer/MultipleDistributionsProfileTest.php
@@ -37,7 +37,7 @@ protected function prepareEnvironment() {
         'distribution' => [
           'name' => $name,
           'install' => [
-            'theme' => 'bartik',
+            'theme' => 'claro',
           ],
         ],
       ];
@@ -57,7 +57,7 @@ protected function setUpLanguage() {
     // Verify that the distribution name appears.
     $this->assertSession()->pageTextContains('distribution_one');
     // Verify that the requested theme is used.
-    $this->assertSession()->responseContains('bartik');
+    $this->assertSession()->responseContains('claro');
     // Verify that the "Choose profile" step does not appear.
     $this->assertSession()->pageTextNotContains('profile');
 
diff --git a/core/tests/Drupal/FunctionalTests/Installer/StandardInstallerTest.php b/core/tests/Drupal/FunctionalTests/Installer/StandardInstallerTest.php
index 7f9c0ae16a935342a2a71f3915221887b8020ebf..d49f373afebc7308d6e5b282b15a1759af07d066 100644
--- a/core/tests/Drupal/FunctionalTests/Installer/StandardInstallerTest.php
+++ b/core/tests/Drupal/FunctionalTests/Installer/StandardInstallerTest.php
@@ -29,7 +29,7 @@ public function testInstaller() {
   protected function setUpSite() {
     // Test that the correct theme is being used.
     $this->assertSession()->responseNotContains('olivero');
-    $this->assertSession()->responseContains('themes/claro/css/theme/install-page.css');
+    $this->assertSession()->responseContains('css/theme/install-page.css');
     parent::setUpSite();
   }
 
diff --git a/core/themes/olivero/olivero.theme b/core/themes/olivero/olivero.theme
index f7a26615e64679b8e9aff5c9d82138f0bee9d4ed..37b29a915b2ab250d40380a545432682e84493aa 100644
--- a/core/themes/olivero/olivero.theme
+++ b/core/themes/olivero/olivero.theme
@@ -23,7 +23,7 @@ function olivero_preprocess_html(&$variables) {
   }
 
   // Convert custom hex to hsl so we can use the hue value
-  $brand_color_hex = theme_get_setting('base_primary_color');
+  $brand_color_hex = theme_get_setting('base_primary_color') ?? '#1b9ae4';
   [$h, $s, $l] = _olivero_hex_to_hsl($brand_color_hex);
 
   $variables['html_attributes']->setAttribute('style', "--color--primary-hue:$h;--color--primary-saturation:$s%;--color--primary-lightness:$l");
@@ -633,7 +633,7 @@ function olivero_form_views_exposed_form_alter(&$form) {
  *
  * @internal
  */
-function _olivero_hex_to_hsl($hex_string) {
+function _olivero_hex_to_hsl(string $hex_string) {
   // Convert hexcode pairs to rgb values (0-255).
   $hex_val = trim($hex_string, '#');
   $r0 = hexdec($hex_val[0] . $hex_val[1]);