Fix faint text in Mermaid architecture diagram

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Problem

The text inside the boxes in the Mermaid architecture diagram in the README is too faint and difficult to read. This affects the readability and accessibility of the documentation.

Context

• PR Convert architecture overview to Mermaid diagram #74 was just created with the initial Mermaid diagram conversion
• The diagram uses light background colors which may be making the text appear faint
• Text needs to be clearly readable while maintaining visual appeal

Requirements

1. Make the text in Mermaid diagram boxes darker for better readability
2. Ensure proper contrast between text and background colors
3. Maintain the color coding scheme but improve text visibility
4. Consider using darker background colors or explicit text color styling

Acceptance Criteria

• Text in all Mermaid diagram boxes is clearly readable
• Proper contrast ratio maintained for accessibility
• Visual appeal of the diagram is preserved
• Color coding scheme remains intact

Technical Notes

The issue is likely caused by:

• Light background colors in the Mermaid styling
• Default text colors that don't provide sufficient contrast
• Missing explicit text color declarations

Priority: Medium
Labels: documentation, enhancement, accessibility