This is definitely because of an elementary OS / Photos specific GTK styling snippet missing in your 3rd party theme. (So not an elementary OS or Photos issue but a theme issue.) I had the same issue and solved it as following:
Copy this snippet into your theme: /.themes (or usr/share/themes) > your theme > GTK-3.18 (or just GTK-3 if there is no version specific folder) > gtk.css AND gtk-dark.css.
I put it after the first closing winged bracket.
... }
/**********
* Photos *
*********/
.checkerboard-layout {
background-color: #383e41;
background-image:
linear-gradient(
45deg,
alpha (
#000,
0.1
) 25%,
transparent 25%,
transparent 75%,
alpha (
#000,
0.1
) 75%,
alpha (
#000,
0.1
)
),
linear-gradient(
45deg,
alpha (
#000,
0.1
) 25%,
transparent 25%,
transparent 75%,
alpha (
#000,
0.1
) 75%,
alpha (
#000,
0.1
)
);
background-size: 24px 24px;
background-position: 0 0, 12px 12px;
}
.checkboard-layout .item {
background-color: #eee;
}
/* end Photos snippet */