Accessibility: Improve the Theme Details modal accessibility.

- sets the modal content to `role="document"` to make screen readers switch back to browse mode
- standardizes the Themes screen and Customizer modals interaction
- Themes screen modal: sets initial focus on the modal overlay instead of the primary button, avoiding to "skip" content for keyboard and screen reader users

Fixes #42055.

Built from https://develop.svn.wordpress.org/trunk@41658


git-svn-id: http://core.svn.wordpress.org/trunk@41492 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/theme.php

@@ -632,7 +632,7 @@ function customize_themes_print_templates() {
 	?>
 	<script type="text/html" id="tmpl-customize-themes-details-view">
 		<div class="theme-backdrop"></div>
-		<div class="theme-wrap wp-clearfix">
+		<div class="theme-wrap wp-clearfix" role="document">
 			<div class="theme-header">
 				<button type="button" class="left dashicons dashicons-no"><span class="screen-reader-text"><?php _e( 'Show previous theme' ); ?></span></button>
 				<button type="button" class="right dashicons dashicons-no"><span class="screen-reader-text"><?php _e( 'Show next theme' ); ?></span></button>