R package bslib: Custom ’Bootstrap’ ’Sass’ Themes for ’shiny’ and ’rmarkdown’. Simplifies custom ’CSS’ styling of both ’shiny’ and ’rmarkdown’ via ’Bootstrap’ ’Sass’. Supports both ’Bootstrap’ 3 and 4 as well as their various ’Bootswatch’ themes. An interactive widget is also provided for previewing themes in real time.