Please add prefix for styles

[Daijobou]

Like jquery ui-* you should add a prefix or a default class/id before mermaid-classes:
https://github.com/knsv/mermaid/blob/master/dist/mermaid.css

.section, .grid, .active, .node, .task is not only used in mermaid ;)

[knsv]

I see the problem. Perhaps more specific selectors for styling as
svg .label
.mermaid .label

in the stylesheet would do the trick.