<type name="Magento\Theme\Model\View\Design">
    <arguments>
        <argument name="themes" xsi:type="array">
            <item name="frontend" xsi:type="string">
                Magento/luma
            </item>
            <item name="adminhtml" xsi:type="string">
                Magento/backend
            </item>
        </argument>
    </arguments>
</type>

This is a type configuration.

This configuration is for the base area. Base area configurations will apply to all the sub-applications.

This configuration tells the Object Manager that when a user wants a Magento\Theme\Model\View\Design object that the following arguments should be used in the foo object's constructor.

• $themes = [...items...];

<type name="Magento\Theme\Model\View\Design">
    <arguments>
        <argument name="storeManager" xsi:type="object">
            Magento\Store\Model\StoreManagerInterface\Proxy
        </argument>
    </arguments>
</type>

This is a type configuration.

This configuration is the application level configuration (`app/etc/di.xml`) fo the object manager. It is not a part of a specific module, but will apply to all sub-applications in Magento 2 (cart, admin, api, cron, etc).

This configuration tells the Object Manager that when a user wants a Magento\Theme\Model\View\Design object that the following arguments should be used in the foo object's constructor.

• $storeManager = new Magento\Store\Model\StoreManagerInterface\Proxy (...)