There's a grayish "box" around Mavaro's small and large size PNG images. The transparency isn't 100% perfect, so less forgiving displays (like Ubuntu 16.04 LTS with GNOME desktop and stock drivers) easily display the problem for the whole world to see.
Thank you for the heads up! We can correct in a future update to the CUP. Could you let us know which Community Use Package you're seeing this in?