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Click on System » Preference » Appearance.  This will open a box which is already on the right window.  Click the Install button near the bottom to add your new theme.  Go through the file system until you select the one you just downloaded.  Assuming that it’s a legitimate theme it will show up in the window you are already looking at.  You can select and customize this theme with your choice of icons, colors, window borders, or whatever you like.  Generally speaking a theme you download already has all these pieces with it to give it a unified look.

If you get a look that you like, you can save it as a customized option so that you don’t have to go back and reset all the settings.  This allows you to quickly switch back and forth between different looks if you choose to do so.