In Gitlab, I've been able to render an Entity Relationship Diagram with Mermaid in a Markdown file as specified here.

This is the Markdown I used:

```mermaid
erDiagram
        CUSTOMER }|..|{ DELIVERY-ADDRESS : has
        CUSTOMER ||--o{ ORDER : places
        CUSTOMER ||--o{ INVOICE : "liable for"
        DELIVERY-ADDRESS ||--o{ ORDER : receives
        INVOICE ||--|{ ORDER : covers
        ORDER ||--|{ ORDER-ITEM : includes
        PRODUCT-CATEGORY ||--|{ PRODUCT : contains
        PRODUCT ||--o{ ORDER-ITEM : "ordered in"
                    
```

The mermaid interactive editor provides an example of configuration:

{
  "theme": "default"
}

But I don't know where to locate that configuration information. I've tried putting it in the same directory, in a file called config.json or mermaid-config.json, but neither of those have worked. I also tried including it in the Markdown which defined the diagram, which only caused it to render incorrectly. Is there a way to specify the theme or other CSS elements for Gitlab?