The Tango Icon Theme is a desktop environment independent set of icons 
following the new icon name specification.

In order to build the package, you need the XML::Simple perl module
and icon-naming-utils, both of which are available from SlackBuilds.org

For XFce, it should work as is: (Settings Manager -> User Interface). 
Note that Xfce's Rodent icon theme inherits Tango icons by default, so some 
of your icons (the ones that are currently falling back to the hicolor theme)
will change to the Tango theme simply by installing it.

KDE users will get slightly better looking icons if they add the option
  --enable-png-creation
to the configure part of the SlackBuild. They can select the icon theme from
within the control center.

To change the default GNOME icon theme execute:

  gconftool-2 --direct --config-source \
  xml:readwrite:/etc/gconf/gconf.xml.defaults --type string \
  --set /desktop/gnome/interface/icon_theme Tango

To change a user-specific GNOME icon theme execute:

  gconftool-2 --type string --set /desktop/gnome/interface/icon_theme Tango

If you are not using any settings manager (fluxbox,blackbox...), you can add 
the following to your .gtkrc-2.0 (create it if it does not exist).

  gtk-icon-theme-name="Tango"