Set Fully Transparent Panel

When you set the panel to be transparent in the default Mint-X theme, you will find that some panel items' backgrounds are not transparent, but you can opt for one of the other themes which come with the transparent background for the panel items. If you prefer the default theme with the items' backgrounds transparent, some tweaks are needed with the steps below:

1. Click Menu and select Terminal.
2. Enter sudo cp -R /usr/share/themes/Mint-X ~/.themes/
3. Enter pluma ~/.themes/Mint-X/gtk-2.0/Apps/panel.rc to open the file with the default text editor.
4. Search for two instances of "Panel/panelbg.png"
5. Comment out the two lines by placing a # at the beginning of the lines, for example: #   bg_pixmap[NORMAL] = ...
6. Save the file.
7. Go to Menu > Applications > Preferences > Appearance, switch to the other theme and then back to the Mint-X theme.

(from techsupportalert.com)