The Menta theme comes with 3 different panel.rc files in
gtk-2.0/widgets folder.

-- panel.rc for light grey  panel background (default)
-- panel-dark-grey.rc for a dark grey panel background
-- panel-black.rc for a black panel background

You simply edit the gtk-2.0/gtkrc file to choose your
favorite background.
Example for choose a black panel background.
Edit those lines:

### EXTERNAL FILES ###
# This includes the file that handles the panels with a light grey background.
include "widgets/panel.rc"
# This includes the file that handles the panels with a dark grey background.
#include "widgets/panel-dark-grey.rc"
# This includes the file that handles the panels with a black background.
#include "widgets/panel-black.rc"

to

### EXTERNAL FILES ###
# This includes the file that handles the panels with a light grey background.
#include "widgets/panel.rc"
# This includes the file that handles the panels with a dark grey background.
#include "widgets/panel-dark-grey.rc"
# This includes the file that handles the panels with a black background.
include "widgets/panel-black.rc"

Than reload the theme.

For getting a full transparancy panel edit this line in
your choosen panel.rc

# Disable (comment out) for normal panel backgrounds or to get a transparancy panel.
  bg_pixmap[NORMAL] = "/Panel/panel-bg.png"

change to

# Disable (comment out) for normal panel backgrounds or to get a transparancy panel.
#	bg_pixmap[NORMAL] = "/Panel/panel-bg.png"

Note: maybe you have to change the text color, depends on you DE background
in your choosen panel.rc edit:

style "theme-panel-text"
{
	fg[NORMAL]	= "#000000"
	fg[PRELIGHT]	= "#000000"
	fg[ACTIVE]	= "#ffffff"
	text[NORMAL]	= "#000000"
	text[PRELIGHT]	= "#000000"
	text[ACTIVE]	= "#ffffff"
}



#############################################
#      CONTRIBUTORS AND SPECIAL THANKS      #
#############################################

Ubuntu art team (This theme wouldn't be possible without them.)
Daniel Foré - http://danrabbit.deviantart.com/
Sean Wilson - http://half-left.deviantart.com/
Roberto - http://zentili.deviantart.com/
Aaron - http://aaron-a-arts.deviantart.com/
Ronjouch - http://ronjouch.deviantart.com/
DeviantART linux/designer community
Gnome-look.org
All the application developers out there. Keep doing what you're doing and listen to the GUI designers. ;-)
Anyone else I may have forgotten. :-)

##############################
#      ABOUT ZUKITWO THEME      #
##############################

The Zukitwo theme was created by Mattias aka lassekongo83 - http://lassekongo83.deviantart.com/
The theme is released under the GPL licence. See the COPYING file for more information.

Zukitwo was inspired by the elementary gtk theme by Daniel Foré.

To use this theme read the included INSTALL file for more information.