If you are a git user, you can get the theme and keep up to date by cloning the repo:
$ git clone https://github.com/dracula/wolfram-notebooks.git
Otherwise, you will need to download the files and unzip them for each release.
The stylesheet is titled "Dracula.nb". Move the stylesheet to folder where your Wolfram Desktop application is installed, and into SystemFiles. Run this code to open the folder:
You may need to restart your application to see the changes take effect.
Get the DraculaTheme resource
Install the stylesheet system-wide with the following:
Receive updates about this theme and more about Dracula.
This theme wouldn't exist without these people. Wanna help too? Check the repository on GitHub.