In order to edit an RStudio theme, the rstheme-files can easily be edited in RStudio itself. However, RStudio does not seem to know the file type ".rstheme" and thus does not apply code highlighting when the theme file is shown in the editor. While the structure of the file is easy to navigate without highlighting, previews of colors are also not shown. Color previews are extremely useful when editing themes.
Currently, I use the following workaround: Copy the theme file with a .css suffix, view and edit this version of the file in the IDE, then save it with the .rstheme suffix before (re-)applying it.
Suggested change/behavior: When opening an rstheme-file in the IDE, apply the same code highlighting and color previews as for css files.