Waylon WalkerMermaid 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
