Example

      %%{init:{"theme":"base", "themeVariables": {"lineColor":"red"}}}%%
      flowchart LR
      subgraph red
      A --> B
      end
    

      %%{init:{"theme":"base", "themeVariables": {"lineColor":"blue"}}}%%
      flowchart LR
      subgraph black
      A --> B
      end
    

      ---
      config:
        theme: base
        themeVariables:
          lineColor: yellow
      ---
      flowchart LR
      subgraph red
      A --> B
      end
    

      ---
      config:
        theme: base
        themeVariables:
          lineColor: green
      ---
      flowchart LR
      subgraph black
      A --> B
      end