Where is my style sheet?

I hope I’m not as stupid as that sounds ….

I’m using the Atahualpa theme at https://www.patiastephens.com/wordpress. I can’t find the correct style sheet.

There isn’t anyplace in the Atahualpa theme options to define my font styles. When I go to the Editor and edit styles.css, it doesn’t have any effect on my styles.

According to the page source and Firebug, my font styles are controlled by https://patiastephens.com/wordpress/?bfa_ata_file=css. I can’t find that file anywhere. Not in Edit Themes and not in my site files.

What am I missing? (And why is this so complicated?)