This covers how I personally was able to make my panel background fully transparent, as well as a small amount of CSS editing for Linux Mint. My current version is 22.2.

In order to mess with the .css of your theme, you need to find where it's located, as well as which specific theme you're using. For me, my themes were located in the /usr/share/themes. There, you should see a bunch of folders named Mint followed by a letter and for most cases, a color. To find which theme you're currently using, what I did was go into my system settings, then into my themes settings. At the bottom, there are advanced settings: you should see to the right names that refer to the theme file name.

Look through the themes folder to find the specific one. In my case, I had Mint-Y-Dark-Purple. Enter the one that corresponds to your folder. Then, right click on the cinnamon folder and click open as root.

Inside, you'll find the cinnamon.css file. First, make a copy of that file and save it somewhere in case you break stuff. You'll need to find the section .panel-top, .panel-bottom, .panel-left, .panel-right. Under background color, change the value to transparent. That specific line in my file was line 420 (nice).

Edit 10/19: Removed chmod 777 advice, replaced with right click open as root advice. I did not know mint had that.