Please be very careful along this tutorial, an error on your code might crash your desktop environment. I used Mint 17.1 on this.

First open the file /usr/share/themes/Linux Mint/cinnamon/cinnamon.css with admin privileges:

$ sudo gedit /usr/share/themes/Linux\ Mint/cinnamon/cinnamon.css

Now hit Ctrl + F and search for "#panel"

Now at the end of this section (panel), add "background-color: rgba(175,175,175,0.05);"

Please note that you can change those values according to your taste (for those who don't know: rgba(<ammount of red, 0-255>, <ammount of green, 0-255>, <ammound of blue, 0-255>, <opacity 0-1>)"

It should be like this (your values may be different from mine):

#panel {
    color: #ffffff;
    background-gradient-start: rgba(70,70,70,0.9);
    background-gradient-end: rgba(50,50,50,0.9);
    background-gradient-direction: vertical;
    font-size: 9pt;
    font-weight: normal;
    height: 25px;
    box-shadow: 0px 0px 2px 2px rgba(0,0,0,0.5);
    background-color: rgba(175,175,175,0.05);
}

Now hit save and restart your PC or simply change to another theme, then back to Linux Mint theme.