Elementary OS Freya provides only 2 themes, the default transparent and Matte, for its Plank dock launcher by default.
For those who want to try more themes, Ken Harkey has made 13 Plank themes into PPA available for Ubuntu 14.04 and Elementary OS Freya. Here are the screenshot for some of them.
How to install these themes:
Open terminal from the launcher, and run below command to add the PPA:
sudo add-apt-repository ppa:justsomedood/justsomeelementary
After that, refresh system cache and install the themes via:
sudo apt-get update sudo apt-get install plank-theme-*
For those who don’t want to add PPA, grab the .deb package for each theme from HERE.
To change your Plank theme, you may install Elementary Tweaks from PPA via blew command, or get it here:
sudo apt-get install elementary-tweaks
Then launch System Settings -> Tweaks -> Plank and select a theme from the drop-down box after Theme: