How to theme mate-panel items?

[gabdub]

• Have you installed GtkInspector?
  GtkInspector allows you to test css changes on the fly (no need to change the theme files)
  If not, check this post: Mate 18.04 - Indicator Applet Complete 1.20.0 - Icons resize issue

• Also check this post: Caja and GTK themes - problem

Hope it helps.