I am running Atahualpa 3.5.3 on an older site, and the following error now always appears at the top of the main page on the website (there is only one static page on this site):

Warning: Use of undefined constant gallery_styles - assumed 'gallery_styles' (this will throw an Error in a future version of PHP) in /homepages/29/d282705311/htdocs/wp-content/themes/atahualpa353/functions.php on line 154

This is a relatively recent development. It did not do this before and nothing has been changed on the site content or theme settings. However...it is likely that the underlying version of PHP has been upgraded since this site was created.

The last version of Atahualpa is 3.7.24

Version 3.5.3 was released on Sep 25, 2010 - over 9 years ago.

I would suggest making a full backup of the site and then upgrading to version 3.7.24 (which was released May 30, 2015 making it over 4 years old)

Atahualpa is no longer being updated because of rule changes back n 2015 by the theme review team. To make the changes they insist on would mean a total rewrite of the theme and the developer (Flynn) has chosen not to do that.