RStudio has recently added a feature that allows users to create custom themes. However, it seems that there is no central place to find those custom RStudio themes. I have just started a Github repository which hosts a collection of user-made RStudio themes.
I have tried to find as many themes as possible via Github and search engines. If something is missing on this list or if you have recently created your own RStudio theme, please feel free to create a pull request or open an issue (or simply send me an e-mail).