Add thumbnail configuration (#861)

* add thumbnail configuration

* pages should be single lines

* Revert "pages should be single lines"

This reverts commit 8cadd05.

Author: ethanpalm

settings.mdx

@@ -112,6 +112,19 @@ See [Themes](themes) for more information.
   </Expandable>
 </ResponseField>
 
+<ResponseField name="thumbnails" type="object">
+  Thumbnail customization for social media and page previews.
+
+  <Expandable title="Thumbnails">
+    <ResponseField name="appearance" type="&quot;light&quot; | &quot;dark&quot;">
+      The visual theme of your thumbnails. If not specified, thumbnails use your site's color scheme defined by the `colors` field.
+    </ResponseField>
+    <ResponseField name="background" type="string">
+      Background image for your thumbnails. Can be a relative path or absolute URL.
+    </ResponseField>
+  </Expandable>
+</ResponseField>
+
 <ResponseField name="styling" type="object">
   Visual styling configurations.