This is something I did for fun, and figured other people might want to have more control over their RStudio experience.
Please create an issue if you have any problems!
Darker Modern Theme
To use this theme, you'll need to do a few things. First, go to "~/RStudio.app/Contents/Resources/www/" and paste custom_styles.css in that directory. Next, paste "index.htm" into the same directory, and allow this file to replace the old "index.htm"