By default, PDF export comes from the rendered default light theme. So, if you switch to Reading view in light mode, you’ll have an approximation of what the exported PDF will look like.
It’s up to theme developers to add rules or options for any specific PDF export colors/fonts/spacing/etc. Some theme settings (heading colors, etc.) can leak through to PDF exports depending on how the theme is put together.
You can also use a @media print export snippet independent of the theme to alter PDF exports. I’ve seen some attempts at dark PDF exports, but the snippets tend to break easily and generally don’t produce the exact results folks are looking for without a lot of tweaking.
There’s one linked here you could try: