By default, in Linux Mint with Cinnamon desktop environment, the system panel is black and opaque. In the settings panel there is no option in the transparency panel, such as in Xfce.
But for the cinnamon environment on the special cinnamon-spices extensions website: https://cinnamon-spices.linuxmint.com/extensions there are many extensions that serve to extend the possibilities of changing the Cinnamon desktop. If you go to this site, you will find that the first in the list of popular extensions is Transparent panels (current time). Installing cinnamon extensions is not required from this site. It is enough to open the System Settings => Extensions and go to the Download tab. If you open this tab for the first time, an additional window will open with an offer to update the cache. Click on the Yes button.
After the cache of extensions is loaded from the site https://cinnamon-spices.linuxmint.com/extensions, select the extension Transparent panels and click on the arrow button to Install.
After installing the extension, go to the settings tab, select Transparent panels and click the plus sign (+) button => Enable.
After enabling the extension, your panel will immediately become transparent with shadows.
The Transparent panels extension is customizable. Click the gear icon to open the settings.
In the Settings, in the General section - Type of transparency, you can choose one of the three types of transparency. And when you activate the Use current theme styles, the panel will become black and opaque again.
May the Force be with you, Good Luck!!! cinnamon-transparent-panels