At the time of writing, there is quite an exciting new development in the world of Shiny UI, which is that developers are starting to provide Shiny interfaces to UI frameworks other than Bootstrap. The first example of this, which will hopefully lead to many more, is an implementation of the Material design framework for Shiny. Material design is the very flat design created by Google in 2014 and familiar to any user of the Android operating system. Here is an example:

The Shiny package itself is based on the open source implementation of Material design, Materialize CSS (materializecss.com/), and is called shinymaterial. It is available on CRAN (cran.r-project.org/web/packages/shinymaterial/index.html). The package itself features different functions from vanilla Shiny, material_radio_button() and material_modal(), for example, but the principles are the same and any Shiny developer should find it easy to use the different functions to give a totally different feel to their application.