Switching Plank's theme it's easy: replace
~/.config/plank/theme. Plank should reflect instantly the changes. However there's a problem: Plank ignores some themes and uses the default one. Well, this happens because the newer version of Plank uses a different notation.
hover.theme on a text editor and paste the contents of the files from the new theme into the existing ones. Now find the words
PlankDrawingDockThemeRenderer and remove the sub-string
Renderer. Don't forget to save.
And that's it! Plank should now apply the new theme. By the way, new themes can be found here.