Mermaid gives us a way to style nodes through the use of css, but rather than using normal css selectors we need to use style <nodeid>.  This also applies to subgraphs, and we can use the name of the subgraph in place of the nodeid.
graph TD;
    a --> A
    A --> B
    B --> C
    style A fill:#f9f,stroke:#333,stroke-width:4px
    style B fill:#f9f,stroke:#333,stroke-width:4px
    subgraph one
        a
    end
    style one fill:#BADA55
produces the following graph
 


 
    
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