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/jabref.git
Download using the GitHub .zip download option and unzip them.
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Open your JabRef Program
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Open the Options/Preferences
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Go to the appearance section
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And select custom theme
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Select the dracula.css file from the zip file
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Press save and enjoy
Note: It may load for a few seconds.