After moving to i3 I’ve installed PCManFM as a GUI file manager. To fix missing icons one has to define an icon theme. These instructions are tested on Ubuntu 12.04, but they must work with other distros.
Install icon theme:
1sudo apt-get install tango-icon-theme*
To apply the theme, create a .gtkrc-2.0 file in the home directory:
1$ cat ~/.gtkrc-2.0
2gtk-icon-theme-name = "Tango"
After this, icons will be enabled.