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/rstudio.git
Download using the GitHub .zip download option and unzip them.
From RStudio click Preferences, Appearance, Add, and then navigate to the rstheme for the theme that you'd like to install. Click apply.
If you're comfortable installing from the console and have Devtools installed then you can copy and run the following in your console:
rstudioapi::addTheme("https://raw.githubusercontent.com/dracula/rstudio/master/dracula.rstheme", apply = TRUE)