Comment 7 for bug 1268871

There is a simple workaround for this issue:
Just go to /usr/share/icons. All your icons should be listed here.
Go into the dir "Numix" for example, there should be a file named index.theme.
Open this file as Administrator with scratch, vi, or whatever texteditor you prefer.
In ther line "Inherits=..." add "elementary," before "gnome"
Usually this line should then look like this:
"Inherits=elementary,gnome,hicolor"

If I'm not mistaken, this should mean that, if an icon is missing, the icon pack "elementary" is searched for this icon.

For me, this fixed the problem with software-center.

This should be applicable for the other icon-packs as well.