Seeing "undefined constant gallery_styles" warning in page display

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.