Update header to white background with black text

[devin-ai-integration]

Summary

Updates the header navigation bar to use a white background with black text instead of the previous red background with white text. This change affects the default "day" theme while maintaining compatibility with the night and amoledblack themes.

Key changes:

• Changed day theme header background from red (#b92b27) to white (#fff)
• Updated header text and navigation link colors from white to black
• Added a new $header-text-color parameter to the theme mixin system for better theme control
• Adjusted logo-inner color to red so it remains visible on the white background

Before

After

Review & Testing Checklist for Human

• Verify the header displays correctly with white background and black text in the default (day) theme
• Test the night theme to ensure header still works correctly (dark background with light text)
• Test the amoledblack theme to ensure header still works correctly
• Check that the HN logo is visible and properly styled on the white header
• Test on mobile viewport to ensure responsive styles are working

Recommended test plan: Run npm start and manually switch between all three themes using the settings cog icon to verify each theme's header renders correctly.

Notes

• The Figma API token provided was invalid, so I was unable to retrieve a screenshot of the reference design. The implementation follows the user's description of "white header with black text."
• A new $header-text-color variable was added to all three themes to properly support the theme system.

Requested by: Joao Esteves (joao.esteves@cognition.ai) / @joao-cognition
Link to Devin run: https://app.devin.ai/sessions/058f0707ef8a47439e5148e5ad8d992f

[devin-ai-integration]

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