This guide shows you how to change plank themes in Elementary OS.
First head over gnome-look.org for some Plank themes: List of plank themes.
Extract the package to .local/share/plank/themes (hidden in your home directory)
Open the terminal type;
You’ll now be greet by this menu.
You can now pick the theme you downloaded. Enjoy!
Tested on: elementary OS 5.1.4 – 5.1.6