Thanks for firing up this issue. After some investigations, I found that it consists of 2 issues:

1. The mermaid stylesheet is outdated. Mermaid now embed styles inline when generating svg. This stylesheet should be removed (from our webpack config).
2. Night mode implementation for mermaid is also outdated. Latest mermaid provides 4 different themes. We should feed in mermaid theme option based on current theme, and then trigger a rerender.
   

   codimd/public/js/extra.js

   Line 397 in 558fa5f

   window.mermaid.init(undefined, $ele)

Ref: https://stackoverflow.com/questions/49535327/change-mermaid-theme-in-markdown