1. Install xdotool and wmctrl:

sudo apt install xdotool wmctrl

2. Save script to ~/bin/ or ~/.local/bin, make it executable and then add it to start-up applications via the startup applications GUI.

3. Log out and log back in.

Enjoy

NB: As written focused-maximised or fullscreen windows are always fully opaque, focused windowed apps are 95% opaque and non focused apps are 70% opaque. You can amend this to suit your preferences.

Credit: smurphos
link 1: https://forums.linuxmint.com/viewtopic.php?t=290071
link 2: https://github.com/smurphos/nemo_actions_and_cinnamon_scripts/blob/master/.local/bin/opacify_windows.sh