[BUG] Mermaid not rendering if using theme

[Zombiefleischer]

Digital Garden is not rendering the mermaid diagram, if the theme is set

Example: %%{init:{'theme':'dark'}}%%

This is my mermaid code and it gets rendered as a white frame with all the code in it

---
title: Client is cluster-aware
---
%%{init:{'theme':'dark'}}%%

flowchart TD
    subgraph Shard3 [Shard 3]
        Master3[Master] --> Replica3[Replica]
    end
    subgraph Shard1 [Shard 1]
        Master1[Master] --> Replica1[Replica]
    end
    subgraph Shard2 [Shard 2]
        Master2[Master] --> Replica2[Replica]
    end

    Client -- Read and Write --> Master1 & Master2 & Master3

---
title: Client is cluster-aware
---
%%{init:{'theme':'dark'}}%%

flowchart TD
    subgraph Shard3 [Shard 3]
        Master3[Master] --> Replica3[Replica]
    end
    subgraph Shard1 [Shard 1]
        Master1[Master] --> Replica1[Replica]
    end
    subgraph Shard2 [Shard 2]
        Master2[Master] --> Replica2[Replica]
    end

    Client -- Read and Write --> Master1 & Master2 & Master3

[cathelijne]

There is an option to do the graph config in yaml, and that works for me. This is an actual gitGraph I'm using that renders fine on my website:

```mermaid
---
config:
  theme: base
  themeVariables:
    git0: "#ff0000"
    git1: "#0000ff"
    commitLabelColor: "#000000"
    commitLabelFontSize: "12px"
    commitLabelBackground: "#ffffff"
  gitGraph:
    showBranches: false
---
gitGraph
  commit id: "Amsterdam " type: HIGHLIGHT
  commit id: "Hiroshima"
  commit id: "Miyajima"
  commit id: "Himeji Castle"
  commit id: "Osaka"
  commit id: "Kyoto"
  commit id: "Nagoya"
  commit id: "Kanazawa"
  commit id: "Takayama"
  commit id: "Matsumoto"
  commit id: "Yokohama"
  commit id: "Tokyo"
  commit id: "Amsterdam"

(it's in a double set of triple backticks to display here, but the preview messes those up a bit, if you already write mermaid diagrams, I'm sure you know how to handle the backticks :))