I tried the theme to see the problem, but i get the new icon of the theme to terminal app and thunar, but for file synaptic, i also get the the old one.
I go to the folder of the theme, and i didn't found icon for synaptic app, so the default one is displayed
i think u can change icon of synaptic by download suitable icon , but i think u should put it in that path
/usr/share/icons/candy-icons/apps/scalable