Receiving XML2 error when trying to add theme

I am very new to using R and R Studios and I am trying to add a custom theme that I downloaded from Herokuapp theme editor. I placed the file that was downloaded into the same folder as all my other R files and when attempting to add the theme from the global options window I receive the above error. This error reads "Unable to add the theme '~/file directory pathway/theme name.tmTheme'. The following error occurred: there is no package called xml2"

I've done plenty of googling and searching of forums to find a possible answer or solution to this problem but I have been unable to find anything worthwhile or anything that mentions my specific problem. As I'm still very new to R & R Studios I have no idea where to even begin fixing this error.

Any help would be greatly appreciated!

I do not know anything about themes.
But why not install the R-package xml2?
It may not help you much, but it will also not hurt.