I've noticed the following lines showing up in my error log with PHP 7.0+:

PHP Warning:  A non-numeric value encountered in /.../wp-content/themes/atahualpa/css.php on line 137
PHP Warning:  A non-numeric value encountered in /.../wp-content/themes/atahualpa/css.php on line 138

I'm using Atahualpa version: 3.7.24.

I searched the forum -- apologies if this is duplicated in any other post. Thank you for all your help over the years.

Please go to ATO->Export/Import Settings - export your settings and attach them to a reply (use the paper clip icon)

Thank you juggledad. See attached Atahualpa settings.

Do you have any clue as to what was going on when the warning shows up?

where are you seeing that message i.e. what error log?

The error_log is generated in Wordpress Debugging Mode.

I think the error is generated when a viewer visits the site.

Do you have define( 'WP_DEBUG', false ); set in your wp_config.php?

I haven't been able to recreate this so I don't know what is causing it. If you can figure out what causes it, I can take a look and come up with a solution.