Mermaid: support dark mode

[chalin]

• Contributes to Support dark mode #331
• Context: @theletterf - Dark mode open-telemetry/opentelemetry.io#4023 (comment)
• https://mermaid.js.org/config/theming.html#available-themes

Tasks

• Format mermaid.html #1963
• Set Mermaid theme based on light/dark mode #1964
• Mermaid: stick to default when not dark mode #1965