I tried different icon sets and the problem remains the same. In particular, I tried with the following icon sets (the ones shipped by default): Crux; Crystal SVG; GNOME; High Contrast Inverse; Mist; Tangerine.
Even with some icon sets taken from gnome-looks (i.e. Bamboo-Zen, Dropline NOU! and Oxygen-refit 2) the problem remains.
In every case, the icon's background color is not transparent, as supposed to be.
I tried different icon sets and the problem remains the same. In particular, I tried with the following icon sets (the ones shipped by default): Crux; Crystal SVG; GNOME; High Contrast Inverse; Mist; Tangerine.
Even with some icon sets taken from gnome-looks (i.e. Bamboo-Zen, Dropline NOU! and Oxygen-refit 2) the problem remains.
In every case, the icon's background color is not transparent, as supposed to be.