If you need to replicate the look of JupyterLab headings manually, you can use JupyterLab's CSS variables. JupyterLab uses --jp-content-font-family for the font family and --jp-content-font-size# (where # is a range from 0 to 5) for the font size.
Warning: Most JupyterLab font size variables use relative em units. This can cause unexpected scaling issues when nesting elements, as the font size will be calculated relative to the parent's size rather than the root.
<h2
style={{
fontFamily: 'var(--jp-content-font-family)',
fontSize: 'var(--jp-content-font-size1)',
}}
>
{'Heading'}
</h2>