How to stack two images horizontally in R Markdown

This might be better broken off into a separate topic, but it seems like a lot of people are intimidated by slide theme customisation using CSS. I wonder if we could jam SASS into the xaringan build process and then have YAML parameters appended to or processed alongside the SASS.

It would mean that you could customise the theme directly from the YAML, and using SASS variables users would only need to make changes to a few variables set up by the theme author...

EDIT: I've moved this topic to its own thread!