I also see this behavior on Obsidian 0.12.15, Windows 10. It’s a shame since I needed to make some notes I’m making with mermaid and them being hard to read is less than ideal.
It’s possible to change mermaid colours, although you’d have to add comments at the end of the mermaid text, and specify which entity you want to change. https://mermaid-js.github.io/mermaid/#/theming
I’ve tried checking if it’s possible to change with css, but I haven’t found a good answer as of yet.