I think the general starting point would be to copy the theme’s styles into print styles (I’m guessing that’s what’s used for PDFs because they’re print-oriented).
Obsidian’s theming system recently changed in a big way. That makes it easier to use, but I don’t know if any of the resources on Obsidian’s CSS have updated to reflect the changes. Still, here they are:
A good resource on HTML and CSS in general (nothing specific to Obsidian): MDN Web Docs
https://forum.obsidian.md/t/getting-comfortable-with-obsidian-css
https://forum.obsidian.md/t/meta-post-common-css-hacks
Recent changes to Obsidian’s CSS:
https://forum.obsidian.md/t/1-0-theme-migration-guide/42537
And an article on print styles: A Guide To The State Of Print Stylesheets In 2018 — Smashing Magazine