Lxappearance is a GUI GTK theme switcher, Gtk deals with the contents of a window, Icons, background window colour (but not the title bar - to configure that see DESKTOP Obconf).
It allows you to choose a gtk+ theme, install and choose icons, select toolbar style and font.
Lxappearance is available in the main menu under Preferences → User Interface settings.
First you need to extract the file you have downloaded, to extract you can use a file manager, simply navigate to the directory where you downloaded your file to (by default ~/downloads), right click on the file and select extract/uncompress. Or in the terminal type (assuming it has a '.tar.gz' extension):
tar xvzf ~/downloads/file_you_have_just_downloaded
for other options see the tar man page:
Then you need to move it to ~/.themes, either using a file manager or the terminal
mv ~/downloads/file_you_have_just_downloaded ~/.themes/
you should now be able to launch Lxappearance and your theme should show up in the list.
To install icons the steps are the same as above, except you need to copy the icons to ~/.icons
Alternatively from within Lxappearance under the 'icon' tab select install, then navigate to where the icons are, note this requires that the icon folder is compressed as either tar.gz or tar.bz2, icons are then stored in ~/.local/share/icons