Did it!

rob@tiranno:~$ ls -la /usr/share/themes/TrueMinimalist/
totale 20
drwxr-xr-x   3 rob  rob  4096 2007-11-25 22:20 .
drwxr-xr-x 125 root root 4096 2007-11-25 22:15 ..
-rw-r--r--   1 rob  rob  1360 2007-11-25 22:20 icon-theme.cache
-rw-r--r--   1 rob  rob   442 2007-10-29 02:39 README
drwxr-xr-x   2 rob  rob  4096 2007-10-29 02:39 xfwm4
rob@tiranno:~$

rob@tiranno:~$ gtk-update-icon-cache -f -t /usr/share/themes/TrueMinimalist/

but it don't want to appear.
ideas?

If they are icons, in an icon pack (icon theme pack)... then you want to place them in /usr/share/icons

Also, icon packs should have sub-folders. In it should be "scalable", "48x48", "24x24", "22x22", and "16x16". In most all of them should be more folders called "actions", "apps", "categories", "devices", "emblems", "emotes", "mimetypes", "places", and "status".

You can just log in to your terminal as root, and then move the icon folder (with all of it's sub-folders) to the proper icons folder. Or you can run "gksu thunar" in alt+f2, and then just drag and drop it there.

Also, if it is a xfwm4 theme (which would be a window border theme), than it would be in the correct folder "/usr/share/themes", but to change the window border you would have to do it here:

Xfce Menu-->--Settings-->--Window Manager Settings

So, just to be clear...

Icons go in /usr/share/icons
xfwm4 window themes go in /usr/share/themes
gtk-2.0 themes go in /usr/share/themes
gdm themes go in /usr/share/gdm/themes

rob@tiranno:~$ ls -la /usr/share/themes/TrueMinimalist/
totale 20
drwxr-xr-x   3 rob  rob  4096 2007-11-25 22:20 .
drwxr-xr-x 125 root root 4096 2007-11-25 22:15 ..
-rw-r--r--   1 rob  rob  1360 2007-11-25 22:20 icon-theme.cache
-rw-r--r--   1 rob  rob   442 2007-10-29 02:39 README
drwxr-xr-x   2 rob  rob  4096 2007-10-29 02:39 xfwm4
rob@tiranno:~$

rob@tiranno:~$ gtk-update-icon-cache -f -t /usr/share/themes/TrueMinimalist/

but it don't want to appear.
ideas?

I've found that the easiest way to install new themes is to do the following. At least in debian based distros, anyway.

First find your themes

Next download them to the desktop.

Next, since they're usually packaged as tarballs extract the files (also to the desktop)

Next open a terminal and use it to open your file manager as root (which, in my case, is sudo thunar)

Navigate to your Themes Folder (in Xubuntu the path is usr/share/themes

Copy the new theme folder and paste it into the Themes Folder

Now, when you open up the User Interface preferences you should see your new theme listed alongside all the others

The same procedure can be used to install new icons, cursors, wallpapers, system sounds, etc. Just remember to navigate to the appropriate folder (in the case of wallpapers this is usr/share/xfce4/backdrops).

I also like to place my themes in the /usr/share/themes folder, and my icons in the /usr/share/icons folder. Just do it as root.

First find your themes

Next download them to the desktop.

Next, since they're usually packaged as tarballs extract the files (also to the desktop)

Next open a terminal and use it to open your file manager as root (which, in my case, is sudo thunar)

Navigate to your Themes Folder (in Xubuntu the path is usr/share/themes

Copy the new theme folder and paste it into the Themes Folder

Now, when you open up the User Interface preferences you should see your new theme listed alongside all the others

The same procedure can be used to install new icons, cursors, wallpapers, system sounds, etc. Just remember to navigate to the appropriate folder (in the case of wallpapers this is usr/share/xfce4/backdrops).

Maybe you also need to logout/login is some cases (or kill xfce-mcs-manager and start it again).

rob@tiranno:~$ gtk-update-icon-cache -f -t ~/.icons/TrueMinimalist/
Cache file created successfully.

but when I seek 'em in the Prefs -> user interf -> icons theme -> ??? I can't find 'em.
Do you know why or what can I do???

I use xfce4 4.3.99.2  under Debian.
bye!