I have changed my Plank theme using elementary Tweaks, but there doesn't exist the default theme option. My question is, how can I get back to the stock Plank theme?
To facilitate switching in the future you can add a default theme to the list:
mkdir -p ~/.local/share/plank/themes ln -s /usr/share/themes/elementary/plank ~/.local/share/plank/themes/Pantheon
After that, the default theme will be named
I got it working. Had to click "revert to default" button.