Missing Navigation Arrows

Hello,
I have the same problem that other users have already reported and I tried the suggested solutions but none work.
I can’t see the navigation arrow left and right.
I am using the latest WordPress installation and the latest version of Foo Gallery
and a free WordPress template called Virtue
https://themes.kadencethemes.com/virtue/

Here you can see the problem:
https://www.associazionelacordata.org/galleria-foto/2017-2/gallery/palestra-di-arrampicata/

Thank you
Laura