Refer: http://tipsonubuntu.com/2018/11/12/make-app-window-transparent-ubuntu-18-04-18-10/
Open terminal either via Ctrl+Alt+T or from software launcher.
When terminal opens, paste following the command and hit enter:
sh -c 'xprop -f _NET_WM_WINDOW_OPACITY 32c -set _NET_WM_WINDOW_OPACITY $(printf 0x%x $((0xffffffff * 80 / 100)))'
Your mouse cursor will become cross-shaped, simply click on any window and it become 80% opacity.
In the command, you can change 80 to number range from 1 ~ 99 for other window opacity.
To reset the window opacity, run command:
sh -c 'xprop -f _NET_WM_WINDOW_OPACITY 32c -set _NET_WM_WINDOW_OPACITY $(printf 0x%x $((0xffffffff * 100 / 100)))'