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"
}

Have fun ;)