This repo is fairly simple. It contains a CSS and Rstudio theme file that make your RStudio editor theme look like Atom One Dark.
In the latest Rstudio version (1.2), you can create your own custom themes without having to overwrite an existing one. Check out the RStudio blog post for an instruction.
If you are using Rstudio version 1.1 and older, you need to overwrite an existing theme (which is a CSS file in the older Rstudio versions). In this case, simply copy the content of atom.css and paste it in the CSS file you want to overwrite.
For an instruction of how to change the RStudio v1.1 theme, read my blog article.