An IntelliJ Darcula-inspired theme for RStudio
With highlighted function calls (Preferences > Code > Display > Highlight R function calls)
Without highlighted function calls
Simply download darculaR.rstheme, add it to your themes (Preferences > Appearance > Editor theme > Add...), and hit Apply!
To make the borders of RStudio dark in addition to the panes (as shown in the above images), use the darkstudio package.
Some helpful resources to look at if you'd like to tweak something or make your own theme: