Index: src/wp-admin/js/theme.js
===================================================================
--- src/wp-admin/js/theme.js	(revision 43019)
+++ src/wp-admin/js/theme.js	(working copy)
@@ -573,16 +573,26 @@
 	// Handles .disabled classes for previous/next buttons in theme installer preview
 	setNavButtonsState: function() {
 		var $themeInstaller = $( '.theme-install-overlay' ),
-			current = _.isUndefined( this.current ) ? this.model : this.current;
+			current = _.isUndefined( this.current ) ? this.model : this.current,
+			previousThemeButton = $themeInstaller.find( '.previous-theme' ),
+			nextThemeButton = $themeInstaller.find( '.next-theme' );
 
 		// Disable previous at the zero position
 		if ( 0 === this.model.collection.indexOf( current ) ) {
-			$themeInstaller.find( '.previous-theme' ).addClass( 'disabled' );
+			previousThemeButton
+				.addClass( 'disabled' )
+				.prop( 'disabled', true );
+
+			nextThemeButton.focus();
 		}
 
 		// Disable next if the next model is undefined
 		if ( _.isUndefined( this.model.collection.at( this.model.collection.indexOf( current ) + 1 ) ) ) {
-			$themeInstaller.find( '.next-theme' ).addClass( 'disabled' );
+			nextThemeButton
+				.addClass( 'disabled' )
+				.prop( 'disabled', true );
+
+			previousThemeButton.focus();
 		}
 	},
 
Index: src/wp-admin/theme-install.php
===================================================================
--- src/wp-admin/theme-install.php	(revision 43019)
+++ src/wp-admin/theme-install.php	(working copy)
@@ -309,8 +309,8 @@
 	<div class="wp-full-overlay-sidebar">
 		<div class="wp-full-overlay-header">
 			<button class="close-full-overlay"><span class="screen-reader-text"><?php _e( 'Close' ); ?></span></button>
-			<button class="previous-theme"><span class="screen-reader-text"><?php _ex( 'Previous', 'Button label for a theme' ); ?></span></button>
-			<button class="next-theme"><span class="screen-reader-text"><?php _ex( 'Next', 'Button label for a theme' ); ?></span></button>
+			<button class="previous-theme"><span class="screen-reader-text"><?php _e( 'Previous theme' ); ?></span></button>
+			<button class="next-theme"><span class="screen-reader-text"><?php _e( 'Next theme' ); ?></span></button>
 			<# if ( data.installed ) { #>
 				<a class="button button-primary activate" href="{{ data.activate_url }}"><?php _e( 'Activate' ); ?></a>
 			<# } else { #>