Based on the original colorscheme rebelot/kanagawa.nvim for neovim.
Edit the file kanagawa.rstheme following the documentation from
https://docs.posit.co/ide/user/ide/guide/ui/appearance.html.
Because RStudio is basically a web browser, you can use the 'inspect element' option to help figure out what setting goes to which object.
I also found this guide to be helpful: https://benharrap.com/post/2024-01-19-rstudio-noodle-theme/
In R Studio, navigate through the menus Tools -> Global Options -> Appearance
then click the 'Add' button and find the file kanagawa.rstheme in your local files.