
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.

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!!!