Refactor Mermaid styles into a theme
Refactor Mermaid styles into either a custom theme, or variable overrides, as documented here: https://github.com/mermaid-js/mermaid/blob/develop/docs/theming.md
Mermaid SVGs include fixed heights and widths on many elements, so we should be passing visual customizations to Mermaid before the diagrams render (via a theme), not after (via custom CSS, as we are now). Making this change should clear up spacing issues we're seeing on some of our diagrams and allow us to more easily support Mermaid diagrams.