RStudio Theme Collection Updates: January 2020

The universe of RStudio themes is growing constantly. I have updated the RStudio Theme Collection to account for new themes that have been created recently.

Thanks to z3tt for the Viridis theme, to William Carcamo for the Custom theme, to Alfredo Hernández for the Darkula theme, to Matt Dube for several themes (Gruvbox, Paper Color), to Linda for Material Theme Darker / darkrstudio, to Alessio Maggiorelli for the Matrix Glow theme, to Stephen Siegert for the Nord Polar Night theme, and to David Gibson for the Ayu themes (dark, mirage, light).