I am trying to create neat summaries of one of my probability courses using R Markdown and I would like to put in evidence some of the important concepts of the course. As an example, I would like to change the background behind important definitions/equations, a bit like in this picture:
You would need to then style the blog according to your need. In HTML it is usually easier with CSS than in LaTeX, unless you are already familliar with LaTeX and available CTAN package that can help.
This is not a direct answer but I believe this is the best options you have in R Markdown feature to create such callout box.
You can also be interested in a new project called Quarto which has built in feature from some callout blocks