I can confirm that silenx's workaround works for me.
"System Settings" -> "Application Appearance" -> "GTK+ Appearance" -> "Widget style" pick one of "Raleigh" or "Oxygen-Molecule"
The former looks out-of-place, while the latter requires you to install the package "oxygen-molecule" from the universe repository.
Because filezilla works correctly with two other themes, would the bug be in the "oxygen-gtk" theme (as opposed to in FileZilla itself)?
I can confirm that silenx's workaround works for me.
"System Settings" -> "Application Appearance" -> "GTK+ Appearance" -> "Widget style"
pick one of "Raleigh" or "Oxygen-Molecule"
The former looks out-of-place, while the latter requires you to install the package "oxygen-molecule" from the universe repository.
Because filezilla works correctly with two other themes, would the bug be in the "oxygen-gtk" theme (as opposed to in FileZilla itself)?