I totally agree, the instant updates in the preview pane are awesome!
I see how it doesn’t look great with the Dark theme 
. Unfortunately, I don’t have the know how or capacity to work on a fix for that. However, I would gladly accept a PR that introduces a solution.
Node styling, as described in https://mermaidjs.github.io/flowchart.html does work. But that may be of limited usefulness in addressing the Dark theme issue.