I've recently installed Rstudio on a new laptop (Linux Fedora, using RPM). I've changed the editor font. However, the "IDE font" (as you can see in the file name) is different from the system font. It looks like Times New Roman (?) and sometimes it's quite difficult to read.
If you can't get what you want from the provided options, there is the option of making a custom theme which allows much more granular access to the settings: