Katex is added as a plugin. You could also add MathJax as a plugin, in which case people could choose between the two.
You only have to make sure that they are mutually exclusive. (I don’t want to know what would happen, if you were to enable both at the same time.)
check out the following 2 files:
ReactNativeClient/lib/MdToHtml.js
ReactNativeClient/lib/MdToHtml/rules/katex.js