First, make sure your version of RStudio is up to date (at least v1.2).
You can install and change your theme directly by running the following line of R code in RStudio:
rstudioapi::addTheme(https://raw.githubusercontent.com/braudm/rstudio_umi_theme/master/Umi.rstheme", apply = TRUE, force = TRUE, globally = FALSE)
Or, you can do it manually through RStudio settings. Download the files and then go to Tools > Global Options > Appearance > Editor theme > Add and select the Umi.rstheme or Umi.tmTheme file. :)