We will need a little more information. Which version of Mint 4 are running; are you using KDE, Gnome, XFCE, or Fluxbox? What theme package(s) did you install?
The reason I'm asking, is that different desktop environments like KDE, Gnome, XFCE, use different window managers. Linux Mint 4 Main and Light use the Gnome desktop environment, which uses the Matacity window manager. If you are also using Compiz desktop effects, you may also be using a window decorator.
Gnome uses Metacity, KDE uses Kwin, and Xfce uses xfwm4. Fluxbox uses a very simple and uses editable text files for themes.
If Compiz is installed, you might be using Emerald for window decoration.
It's not as complicated as it sounds. but this is what you get when you have virtually limitless control over your operating system.
Try opening Synaptic and review the installed packages. The name or description of the package should give you an idea of a what's been installed, and if you can use it.
Instructions for LM4 Main, Using Gnome and Matacity:
Find a site like http://www.gnome-look.org
. Choose Matacity from the left-hand side of the site, and browse for a theme you like. once you save the zip file you can simple drag and drop it to install. Open the Mint Menu -> Preferences -> Appearance. Make sure the "Theme" tab is selected, then drag your zip file to the window. If the theme was packaged correctly, you'll notice a prompt acknowledging that new themes have been installed.
I hope this helps, I've tried to cover as many bases as possible to help you with your problem. I been Linux only for about a year, so I hope that others might chime in here and correct me or contribute to the help.
Please follow up and let me know if this has helped.