Thanks Kate!
My confusion was with the wording on the settings page I guess. When I read that your theme must have “custom css”, I read that as having the WordPress feature of Appearance > Edit CSS.
I added a new file named custom.css to my themes folder and it is working now.
I’d recommend a small text update on the settings page to make that a little more clear with a future update.
Great plugin by the way!
Thanks,
Brandon