Documents produced with the beamer class look different than the example Marker/test/beamer.pdf.
Most importantly, --- does not trigger a new slide. Here is a minimal example based on
Marker/test/beamer.md to illustrate what I mean.

Here is the source file and how it is displayed in user interface:

Exporting the document to PDF produces a document with a single page:

I have activated the beamer css theme under Preferences/Preview/Use css theme. I run marker on Fedora 32. The problem occurs both in the flathub version (2020.04.04), as well as with my own compilation of the most recent git version (commit 9c20e11bfbb9dacc1be58b745b107a7939b47582).