Themes are specific to GNOME and KDE.
In gnome there are two types of themes: WIndow border (Window manager), Application. Make sure you have the correct type of theme for the section of the theme manager you are in. The theme ".tar.gz" files for most Metacity (GNOME window manager) themes start with "MCITY-" and most GTK ones begin with "GTK2-"
See
Gnome Art and look under the GTK2 and metacity sections for themes. Just drag the "tar.gz"or "tar.bz2" file from nautilus to the correct tab on the theme manager.
In kde, just go to
Kde look.
If that doen't work, thereis something wrong with the theme manager.