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

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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