Thanks for checking.
I just checked as well and on my system (arch, xfce) the file exists but it's a symlink, created by a theme I tried but doesn't exist on my system anymore ("qogir-gtk-theme-git"). After removing the symlinks it works.
I clicked apply in the app and a gtk.css and settings.ini files was created. The gtk.css file it's empty. Is that the intended behavior?
A list of features would be nice.
Is the calendar widget part of the launcher for example?