Visit Ulysses Style Exchange and click
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/ulysses.git
Download using the GitHub .zip download option and unzip them.
Ulysses > Preferences...
Draculain the list of themes at the bottom.
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This theme wouldn't exist without these people. Wanna help too? Check the repository on GitHub.