Hi I was a little confused in my last reply, sorry.
If you want the dark theme; just set your IDE theme to Modern, and select a dark Editor theme, for example dracula. The IDE will switch between dark and light to match.
A few pro tips to further custimize your theme, check out the full instructions at the link below, and in particular check out the "Making and Sharing Themes" section
For an example of a really cool looking custom theme, check out RStudio IDE Dark Theme for Apple Lovers.