docs(stylesheets): resolve lines and arrows visibility in dark mode

  --md-mermaid-custom-secondaryTextColor: #ffffff;

  --md-mermaid-custom-tertiaryColor: #e2f0ff;

  --md-mermaid-custom-tertiaryBordColor: #00000000;

  --md-mermaid-custom-tertiaryTextColor: #222222;

  --md-mermaid-custom-tertiaryTextColor: #004f77;

  --md-mermaid-edge-color: var(--md-mermaid-custom-tertiaryTextColor);

  --md-mermaid-node-bg-color: var(--md-mermaid-custom-primaryColor);