Index: src/wp-admin/js/theme.js
===================================================================
--- src/wp-admin/js/theme.js	(revision 42832)
+++ src/wp-admin/js/theme.js	(working copy)
@@ -577,12 +577,16 @@
 
 		// Disable previous at the zero position
 		if ( 0 === this.model.collection.indexOf( current ) ) {
-			$themeInstaller.find( '.previous-theme' ).addClass( 'disabled' );
+			$themeInstaller.find( '.previous-theme' )
+				.addClass( 'disabled' )
+				.prop( 'disabled', true );
 		}
 
 		// 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' );
+			$themeInstaller.find( '.next-theme' )
+				.addClass( 'disabled' )
+				.prop( 'disabled', true );
 		}
 	},