只需将以下代码添加到主题文件的<header> </header>之间就行了

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    <!-- The loading of KaTeX is deferred to speed up page rendering -->
    <script defer src="https://cdn.jsdelivr.net/npm/katex@0.12.0/dist/katex.min.js" integrity="sha384-g7c+Jr9ZivxKLnZTDUhnkOnsh30B4H0rpLUpJ4jAIKs4fnJI+sEnkvrMWph2EDg4" crossorigin="anonymous"></script>

    <!-- To automatically render math in text elements, include the auto-render extension: -->
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        onload="renderMathInElement(document.body);"></script>
<script>
    renderMathInElement(document.body,
   {
              delimiters: [
                  {left: "$$", right: "$$", display: true},
                  {left: "$", right: "$", display: false}
              ]
          }
  );
</script>

效果：
$$x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$$

$$ \left( \begin{matrix} 1 & x & x^2 \\ 1 & y & y^2 \\ 1 & z & z^2 \\ \end{matrix} \right) $$

$$ X=\left| \begin{matrix} x_{11} & x_{12} & \cdots & x_{1d}\\ x_{21} & x_{22} & \cdots & x_{2d}\\ \vdots & \vdots & \ddots & \vdots\\ x_{m1} & x_{m2} & \cdots & x_{md}\\ \end{matrix} \right| $$

$$ f(n) = \begin{cases} n/2 & \text{if $n$ is even} \\ 3n+1 & \text{if $n$ is odd} \end{cases} $$

$$ \left\{ \begin{array}{c} a_1x+b_1y+c_1z=d_1 \\ a_2x+b_2y+c_2z=d_2 \\ a_3x+b_3y+c_3z=d_3 \end{array} \right. $$

$$ J(\theta)=\frac1{2m}\sum_{i=0}^m(h_\theta(x^i)-y^i)^2 $$

$$\alpha+\beta=\gamma$$

$$ \left[ \begin{matrix} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 9 \end{matrix} \right] $$

官方教程: https://katex.org/docs/browser.html