Edit the file default theme's cinnamon.css with administrator's rights:
sudo xed /usr/share/themes/Linux\ Mint/cinnamon/cinnamon.css
When you open the text editor with the file cinnamon.css, press Ctrl+F in the search box copy/paste the #panel, and then click Find:
Now at the end of this section (panel), add the line (copy/paste):
Save the file and close the editor.
To make the changes take effect, you must restart Cinnamon. Press Alt+F2, type r and press Enter.
Now the system panel Cinnamon should be transparent.