Describe the bug
If you defined themeVariables to customize colors, fonts, etc., the customizations are only applied to the first diagram in the Markdown document. The second diagram is rendered with the default theme.

To Reproduce

1. Setup themes variables:

    remarkPlugins: [
      [
        require("mdx-mermaid"),
        {
          mermaid: {
            theme: "base",
            themeVariables: {
              primaryColor: "lime",
            },
          },
        },
      ],
    ],
   

2. Add two diagrams to a Markdown file:

   ```mermaid
   graph TD;
        A-->B;
        A-->C;
        B-->D;
        C-->D;
    ```
   
    ```mermaid
    graph TD;
        A-->B;
        A-->C;
        B-->D;
        C-->D;
    ```
   

3. Build with yarn start
4. See two diagrams with different colors. (see screenshot)

Expected behavior
All diagrams should share the same theme configuration.

Screenshots

Stack trace
N/A

Tool chain used:
Docusaurus 2.0.0-beta.9, mdx-mermaid 1.2.1

Additional context
When experimenting with different colors, you have to CTRL-C to break yarn start and then run it again to see the changes.