After upgrading Rstudio 1.2.1335 my .R script and .Rmd markdown files are displaying like text. I've checked my options in global options. Restoring the previous version of Rstudio causes the problem to go away and reinstalling 1.2 brings it back. I am running on Mac OS Mojave 10.14.6 on a 2017 iMac Retina 5k 27 inch any thoughts?
I had a similar issue a time ago after updating RStudio and got fixed by temporarily changing the theme to any other option, applying changes and then changing back to my previous theme.
I'm not sure if this is going to solve your problem but you can give it a try.