How to use bootstraplib's Live Theme Previewer to customize Shiny apps?

One of the announcements of RStudio conf 2020 that caught my eyes is a brand new package {bootstraplib} - https://github.com/rstudio/bootstraplib/ . It’s another open-source contribution from RStudio (a PBC). {bootstraplib} basically provides tools for theming shiny and rmarkdown from R via Bootstrap (3 or 4) Sass. If you’re not aware of Bootstrap, it’s one of the most popular (open-source) css framework available on the web. While {bootstraplib} has got a lot of things, this post is going to be about one thing that I love the most which is “Live Theme Preview” where we can simply edit the values of CSS properties and see the change in Real time.