Add dark mode theme support for Mermaid diagrams

Gitiles renders Mermaid diagrams server-side into SVG. Previously, all
SVG shapes (nodes, subgraphs, edges, markers, text, and labels) used
hardcoded light-mode hex values, causing them to display as glaring
white rectangles with poor contrast in dark mode.

This change:
- Defines CSS variables for Mermaid diagram elements (--mermaid-node-bg,
  --mermaid-node-stroke, --mermaid-node-text, --mermaid-edge,
  --mermaid-subgraph-bg, etc.) in doc.css under :root, light mode,
  dark media query, and html[data-theme="dark"].
- Adds semantic CSS classes (.mermaid-node, .mermaid-subgraph,
  .mermaid-edge, .mermaid-arrow, .mermaid-shadow, etc.) to rendered SVG
  elements in SimpleMermaidRenderer while preserving presentation
  attribute fallbacks.
- Selectively applies fill/stroke classes when custom style directives
  are absent, preserving user-defined colors without stylesheet overrides.
- Supports `color:` in `style` directives and computes background
  luminance for custom-filled nodes (e.g. pastel database cylinders)
  to preserve dark, readable text without stylesheet theme inversion.
- Adds comprehensive unit tests in SimpleMermaidRendererTest covering
  theme classes, custom styles, and text contrast on custom-filled shapes.

Change-Id: Iac234a2dd627a06ecceb41c4fabbbbdd92233605