Themes: Use consistent thumbnail sizes in Customizer preview screen.

This ensures displaying theme thumbnails in consistent sizes in Customizer theme preview.

Props poena, karpstrucking, costdev, audrasjb.
Fixes #54188.

Built from https://develop.svn.wordpress.org/trunk@53191


git-svn-id: http://core.svn.wordpress.org/trunk@52780 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/theme-install.php

@@ -456,7 +456,9 @@ if ( $tab ) {
 						?>
 					</span>
 
-					<img class="theme-screenshot" src="{{ data.screenshot_url }}?ver={{ data.version }}" alt="" />
+					<div class="theme-screenshot">
+						<img class="theme-screenshot" src="{{ data.screenshot_url }}?ver={{ data.version }}" alt="" />
+					</div>
 
 					<div class="theme-details">
 						<# if ( data.rating ) { #>