Apply styles from inside mermaid graphs

[micartey]

You can set styles inside a mermaid graph with something like:

subgraph container [Container]
    ....
    style container fill: transparent, stroke-width: 0px, padding: 0px, margin: 0px
end

But the style is not applied when building the graph to a png e.g.
I saw that there is a -cssFile flag...

Now I see two possibilities:

1. Create CSS files - If so, then how can I set the style for the container subgraph?
2. The CLI should detect the styles and apply them on the go