This quick tutorial is going to show you how to make any application window in Ubuntu Gnome transparent.
Since Ubuntu 18.04 switched to Gnome 3 desktop, you can no longer set transparent application window with CCSM utility.
As a workaround, you can run following steps to set any application window opacity in Ubuntu 18.04, and Ubuntu 18.10.
1. Open terminal either via Ctrl+Alt+T or from software launcher.
2. 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.
3. 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)))'
When mouse cursor changes, click on the window to reset its transparent level.