Like
digraph { node [style=filled,color=white]; a -> b }
The result is:

On viz-js.com , it seems :

I know why!.
The mermaid we use is older version, it's css file 'mermaid.forest.css' override the node's style.
So, If we upgrade the mermeiad, we do not need the mermaid.forest.css file. So it is ok.
Like this:
