Index: src/wp-admin/themes.php
===================================================================
--- src/wp-admin/themes.php	(revision 27115)
+++ src/wp-admin/themes.php	(working copy)
@@ -312,9 +312,9 @@
 	<div class="theme-backdrop"></div>
 	<div class="theme-wrap">
 		<div class="theme-header">
-			<button alt="<?php _e( 'Show previous theme' ); ?>" class="left dashicons dashicons-no"></button>
-			<button alt="<?php _e( 'Show next theme' ); ?>" class="right dashicons dashicons-no"></button>
-			<button alt="<?php _e( 'Close overlay' ); ?>" class="close dashicons dashicons-no"></button>
+			<button class="left dashicons dashicons-no"><span class="screen-reader-text"><?php _e( 'Show previous theme' ); ?></span></button>
+			<button class="right dashicons dashicons-no"><span class="screen-reader-text"><?php _e( 'Show next theme' ); ?></span></button>
+			<button class="close dashicons dashicons-no"><span class="screen-reader-text"><?php _e( 'Close overlay' ); ?></span></button>
 		</div>
 		<div class="theme-about">
 			<div class="theme-screenshots">