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/gajim.git
Download using the GitHub .zip download option and unzip them.
If this is not enough you also need to:
Gajim > Preferences > Style > Theme
Get updates about this theme and more. 5,986 people enjoy it.
This theme wouldn't exist without these people. Wanna help too? Check the repository on GitHub.