title: how to set up theorem environment in the Rmarkdown presentation
body: <p>I am new to Rmarkdown and plan to use ioslides/slidy/xaringan to do my presentation.</p>
  <p>I used to use the beamer to do presentations. In the beamer, I have theorem environment which is designed to mathematics theorems. I want to be able to have that kind of format in ioslides/slidy/xaringan. I know I can use $$...$$ to include latex code and can display equation. However, this is not enough to my needs.</p>
  <p>I also know in bookdown one can have theorem environment. But I do not know how to do that in the
  ioslides/slidy/xaringan output format.</p>