Hi Nick,
You’re welcome to report any issue you encounter with Dune and its snippets here or on my github page
I placed Dune-mermaid.css in a folder called snippets and so I assumed everyone interested would know where to place this file in Obsidian. And yes, all snippets need to be enabled to work properly. Here’s the help page how to install snippets : CSS snippets - Obsidian Help
The style settings plugin does all the magic, Obsidian doesn’t know other colors than those default ones. I understand it may be confusing if a snippet applies a theme. Once mermaid gets a proper update, I’ll add it back to my Dune theme.