How to enable the GTK+ inspector on 18.04?

[lah7]

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Enabling the GTK Inspector - to find CSS values for theme styles

When developing or modifying a theme, the GTK Inspector is a useful utility to find out the CSS classes used by a particular theme. Applies to 18.04 and later. Inspection only works on GTK3 applications. Assumes basic knowledge of GTK themes. First, install development files for GTK3: sudo apt install libgtk-3-dev Set an environment variable for the terminal session: export GTK_DEBUG=interactive Tip: To inspect Caja, hidden windows and some a…