I needed a dark Rstudio theme that didn't look like an early 2000s application, but like my code to really glow. So I mashed together two Rstudio themes that I liked to keep the best of both. I have mixed here the UI elements from the excellent Horizon-Dark theme, as well as the text settings from the Synthwave85 theme. I did modify the background color for the code blocks to make it blacker.
The Original Themes were:
The rsthemes package should be setup and the default themes installed.
devtools::install_github("gadenbuie/rsthemes")
rsthemes::install_rsthemes()
Download the raw theme file and open Rstudio.
In the settings menu, select Appearance -> Add and select the downloaded
theme