This theme supports rendering beautiful math in inline and display modes using MathJax 3 engine. You just need to surround your math expression with $$, like $$ E = mc^2 $$. If you leave it inside a paragraph, it will produce an inline expression, just like $$E = mc^2$$. To use display mode, again surround your expression with $$ and place it as a separate paragraph. Here is an example:

$\sum_{k=1}^\infty |\langle x, e_k \rangle|^2 \leq \|x\|^2$

You can also use $$...$$ instead of  for display mode math. MathJax will automatically number equations:

$$\label{eq:caushy-shwarz} \left( \sum_{k=1}^n a_k b_k \right)^2 \leq \left( \sum_{k=1}^n a_k^2 \right) \left( \sum_{k=1}^n b_k^2 \right)$$

and by adding \label{...} inside the equation environment, we can now refer to the equation using \eqref.

Note that MathJax 3 is a major re-write of MathJax that brought a significant improvement to the loading and rendering speed, which is now on par with KaTeX.