Add the ability to manually select the active admin theme

Model/Config/Source/AdminThemeList.php

@@ -0,0 +1,52 @@
+<?php declare(strict_types=1);
+
+namespace MageOS\ThemeAdminhtmlSwitcher\Model\Config\Source;
+
+use Magento\Framework\App\Area;
+use Magento\Framework\Data\OptionSourceInterface;
+use Magento\Framework\View\Design\Theme\ThemeList;
+
+class AdminThemeList implements OptionSourceInterface
+{
+    private ThemeList $themeList;
+
+    /**
+     * AdminThemeList constructor.
+     *
+     * @param ThemeList $themeList
+     */
+    public function __construct(ThemeList $themeList)
+    {
+        $this->themeList = $themeList;
+    }
+
+    /**
+     * Get a list of all installed adminhtml themes.
+     *
+     * @return array
+     */
+    public function toOptionArray(): array
+    {
+        $themes = [];
+        $this->themeList->addConstraint(ThemeList::CONSTRAINT_AREA, Area::AREA_ADMINHTML);
+
+        foreach ($this->themeList->getItems() as $theme) {
+            $path = $theme->getData('theme_path');
+
+            // Replace default admin theme title as 'Magento 2 backend' is not user friendly
+            $title = $theme->getData('theme_title');
+            if ($path === 'Magento/backend') {
+                $title = (string)__('Magento Default');
+            }
+
+            $themes[$title] = [
+                'label' => $title . ' (' . $path . ')',
+                'value' => $path
+            ];
+        }
+
+        ksort($themes); // Sort themes alphabetically
+
+        return $themes;
+    }
+}

Plugin/ThemeAdminhtmlSwitcherPlugin.php

@@ -3,13 +3,12 @@
 namespace MageOS\ThemeAdminhtmlSwitcher\Plugin;
 
 use Magento\Framework\App\Config\ScopeConfigInterface;
-use Magento\Store\Model\ScopeInterface;
 use Magento\Theme\Model\View\Design;
 use Magento\Framework\App\Area;
 
 class ThemeAdminhtmlSwitcherPlugin
 {
-    private $scopeConfig;
+    private ScopeConfigInterface $scopeConfig;
 
     /**
      * ThemeAdminhtmlSwitcherPlugin constructor.
@@ -40,15 +39,11 @@ public function beforeSetDesignTheme(Design $subject, $themeId = null, $area = n
             return [$themeId, $area];
         }
 
-        $isEnabled = $this->scopeConfig->isSetFlag(
-            'admin/system_admin_design/enable_theme_adminhtml_m137',
+        $activeTheme = $this->scopeConfig->getValue(
+            'admin/system_admin_design/active_theme',
             ScopeConfigInterface::SCOPE_TYPE_DEFAULT
         );
-
-        if ($isEnabled) {
-            $themeId = 'MageOS/m137-admin-theme';
-        }
-
-        return [$themeId, $area];
+
+        return [$activeTheme ?? $themeId, $area];
     }
 }

etc/adminhtml/system.xml

@@ -1,13 +1,13 @@
 <?xml version="1.0"?>
 <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-       xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/system_file.xsd">
+        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
     <system>
         <section id="admin">
             <group id="system_admin_design" translate="label" type="text" sortOrder="2100" showInDefault="1">
                 <label>Admin Design</label>
-                <field id="enable_theme_adminhtml_m137" translate="label" type="select" showInDefault="1" showInWebsite="0" showInStore="0">
-                    <label>Enable M137 Admin Theme</label>
-                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
+                <field id="active_theme" translate="label" type="select" showInDefault="1" showInWebsite="0" showInStore="0" canRestore="1">
+                    <label>Active Admin Theme</label>
+                    <source_model>MageOS\ThemeAdminhtmlSwitcher\Model\Config\Source\AdminThemeList</source_model>
                 </field>
             </group>
         </section>

etc/config.xml

@@ -1,10 +1,10 @@
 <?xml version="1.0"?>
 <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-       xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/config_file.xsd">
+        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
     <default>
         <admin>
             <system_admin_design>
-                <enable_theme_adminhtml_m137>1</enable_theme_adminhtml_m137>
+                <active_theme>MageOS/m137-admin-theme</active_theme>
             </system_admin_design>
         </admin>
     </default>

i18n/en_US.csv

@@ -1,2 +1,3 @@
 "Enable M137 Admin Theme","Enable M137 Admin Theme"
 "Admin Design","Admin Design"
+"Magento Default","Magento Default"