By default in Linux Mint 18.x Cinnamon theme in the desktop system panel is black and opaque. In the settings panel there is no option or setting the transparency panel. But in Linux file system is open for user and administrative changes that where in configuration files it is possible to make the necessary changes. Something that is not provided by the developers, we will try to do it yourself.
Edit the file default theme’s cinnamon.css with administrator’s rights:
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.