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
Also, be sure you have all of the latest packages from /patches on
the Slackware mirror installed, especially gnome-icon-theme.

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.

GNOME and KDE users need to do some changes.

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

KDE users need to perform further steps. 
First, edit /usr/share/icons/Tango/index.theme:

  sed -i "s/Context=\w*/Context=FileSystems/g" index.theme

and place a symlink to Tango either in your user specific or KDE's default 
location (see /opt/kde/bin/kde-config --path icon).  You should then be able 
to select the icon theme from within the control center.

KDE users will get slightly better looking icons if they add the option
  --enable-png-creation
to the configure part of the SlackBuild.

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"