How to make windows transparent by default

NOTE: For those who interested on getting transparent window, you might want to install Compiz Settings and its plugins:

sudo apt install compizconfig-settings-manager compiz-plugins

1. Press Alt + F2, type ccsm on field to launch Compiz Setting.

2. On Accessibility section, click Opacity Brightness & Saturation plugin.

3. On the current tab, expand Window Specific Settings.

4. Click New button, then click + and Edit Match window will be appear.

5. On Type field, change value from "Window Class" to "Window Type". Then type "Normal" on Value field. Don't forget to click Add when you're done.

6. Back to edit window, you have to set your preferred opacity values, you can set the values from 0 to 100. Less values make the window more transparent, we'll use 80 for example.

7. Click Close then, as you can see there are new defined specific settings.

Before effect

After effect

Does not work any more on Ubuntu 18.04 and later, because ccsm works only on Unity desktop environment, and latest versions switched to GNOME.

As this site says, you can enter this line on your terminal:

xprop -f _NET_WM_WINDOW_OPACITY 32c -set _NET_WM_WINDOW_OPACITY `printf 0x%x $((0xfffffff * 80 / 100))`

Of course you can replace 80 in the command with whatever value you want, in percentage.

Then the cursor will become a cross, and you can click a window to change its opacity.

NOTE: You have to run this command for every window you have open or you wish to apply that opacity setting to. Once a window is clicked, the cursor becomes back normal.