tidytemplate provides a custom pkgdown template for core tidyverse packages (i.e. packages hosted by the tidyverse organisation). Please don't use it for your own package.
The theme is built on top of the paper bootswatch theme.
To regenerate tidyverse.css, first install sass, then run:
# gem install sass
sass scss/tidyverse.scss inst/pkgdown/assets/tidyverse.css