Popover only works with Luma

Added callout to explains that the popover only works with the Luma theme, or with customized themes derived from Luma.  Added a new section with a section of code that must not be modified.

Author: ccondra

src/live-search/storefront-popover.md

@@ -7,6 +7,11 @@ When Live Search is installed, a popover appears in the storefront as shoppers t
 
 Live Search returns results for a query of two characters or more. For a partial match, the maximum number of characters per word is 20. The number of characters in a "search as you type" query is not configurable.
 
+{:.bs-callout-info}
+The Live Search storefront popover is available only for stores that use the _Luma_ theme, or a customized theme that is based on _Luma_. The _Luma_ theme is included in the Commerce sample data. The popover does not support the _Blank_ theme. See [Working with a modified theme](#working-with-a-modified-theme) for more information.
+
+## Searchable attributes
+
 The following attributes are always searchable:
 
 -  `sku`
@@ -81,6 +86,20 @@ _.livesearch.product-name_
 ![Product price]({{ page.baseurl }}/live-search/images/livesearch-product-price.png)
 _.livesearch.product-price_
 
+## Working with a modified theme
+
+The storefront popover can be used with a customized [theme](https://devdocs.magento.com/guides/v2.3/frontend-dev-guide/themes/theme-overview.html) that inherits the required files from _Luma_. The `top.search` block in the `header-wrapper` of the `Magento_Search` module must not be modified.
+
+```html
+<referenceContainer name="header-wrapper">
+   <block class="Magento\Framework\View\Element\Template" name="top.search" as="topSearch" template="Magento_Search::form.mini.phtml">
+      <arguments>
+         <argument name="configProvider" xsi:type="object">Magento\Search\ViewModel\ConfigProvider</argument>
+      </arguments>
+   </block>
+</referenceContainer>
+```
+
 ## Disabling the popover
 
 To disable the popover and restore standard [Quick Search](https://docs.magento.com/user-guide/catalog/search-quick.html) functionality, enter the following command: