Here is another How-To on GTK/libpeas plugin development. This time it is on how to use the GTK Inspector to dive into application details.

How to enable GTK Inspector

Before we can start we have to enable the GTK inspector with 
gsettings set org.gtk.Settings.Debug enable-inspector-keybinding true
Now with any GTK application you can open the GTK inspector by pressing either Ctrl-Shift-I or Ctrl-Shift-D. If this doesn't help try launching with 
GTK_DEBUG=interactive liferea

Discovering widget names with GTK Inspector

When you have successfully launched the inspector you should see a window like this: Note the second column 'Name' of the "Objects" table containing all those widget names we covered in part 2/3 of the tutorial.

Trying GTK CSS styles

If you select the 'CSS' tab you get a inline editor to try style changes in Liferea. If you make modifications successfully you can save them to disk and make them permanent. For quick effect try entering 
treeview {
   background-color:red;
}

button {
  font-size: 200%
}
and notice the immediate changes. To address widgets by name ensure to always prefix the widget type selector. For example to change the 'itemlist' treeview do 
treeview#itemlist {
	background-color:green;
}
and not just (as one would in HTML) 
#itemlist {
	background-color:green;
}
You probably also noticed that widget type names are not 'GtkTreeView' but 'treeview', not 'GtkButton' but 'button' and so on. Just lowercase and strip 'Gtk' and you have the selector name.

Experiment with GTK settings

Finally check out the 'Visual' tab which exposes all major GTK settings on a per application basis. Here you can try fonts, theme setting, dark variant...

Related

Also check out the previous plugin tutorial posts