One tool that I find invaluable is the gtk-inspector tool.
Gtk-Inspector comes with Zorin OS. You only need to enable it.
gsettings set org.gtk.Settings.Debug enable-inspector-keybinding true
To launch it, run from terminal. Use the GTK_DEBUG=interactive prefix followed by the application name you wish to launch and inspect. For example, Nautilus File Manager:
You should see Two Windows launch.
Nautilus... of course...
And the gtk-inspector window. It has several useful tabs at the top that you can explore.
But for now, let's zero in on using it for what you want to inspect.
Far upper left is a Magnifying Glass icon. Click that puppy. The mouse cursor now is a target selector. Hover over and click on the portion of the window you want css information on.
Hovering over a given area will highlight that area. You can narrow it down to an image-button on a pathbar - or highlight the entire pathbar.
Once selected, go back to the inspector window.
Upper left, you will see a drop down menu marked Miscellaneous.
Click the drop down and change that to CSS Nodes
In the left pane, you will see the Class Tree.
In the right pane, you will see the parameters, what value they have and it will even show the relevant Line that is on in the gtk.css file that is being referenced to govern that value.
It looks a bit daunting at first, but if you play with it like your new favorite toy for a while, learning it is easy.