Head over to the Panic website for the one-click install.
If you are a git user, you can install the theme and keep up to date by cloning the repo:
$ git clone https://github.com/dracula/coda.git
And creating a symbolic link to Coda's theme folder:
$ ln -s coda/Dracula.sss ~/Library/Application Support/Coda 2/Styles/Dracula.sss
Dracula.sssfile to Coda's theme folder:
~/Library/Application Support/Coda 2/Styles/Dracula.sss.
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This theme wouldn't exist without these people. Wanna help too? Check the repository on GitHub.