Update header to white background with black text to match Figma design

[devin-ai-integration]

Update header to white background with black text to match Figma design

Summary

This PR updates the header navigation styling to match a specific Figma design (node 1-3258), changing from a red background with white text to a clean white background with black text. The changes include:

• Header background: Red (#b92b27) → White (#fff)
• Navigation text: White → Black with proper contrast ratios
• Border styling: Prominent red border → Subtle gray border (1px solid #e0e0e0)
• Logo background: White → Light gray (#f5f5f5)

Only the day theme is affected; night and amoled themes remain unchanged.

Review & Testing Checklist for Human

• Visual comparison: Compare the live result with the Figma design to ensure exact match
• Theme verification: Test night and amoled themes to confirm they weren't accidentally affected by the variable changes
• Mobile responsiveness: Verify header looks correct and is readable on mobile devices
• Cross-browser testing: Test in different browsers (Chrome, Firefox, Safari) to ensure consistent appearance

Test Plan

1. Run the app locally (npm start)
2. Navigate through different pages to verify header consistency
3. Toggle between different themes in settings
4. Test on mobile viewport/device
5. Verify navigation links are clickable and hover states work properly

Notes

The changes only modify SCSS styling files and don't affect application logic. Screenshots showing before/after comparison are available for visual verification.

Link to Devin run: https://app.devin.ai/sessions/2d814f58a58b46cd9e395e1093a2a39d
Requested by: Luke Page (@lukeepage2)

Screenshots

Figma Design:

Before (Red Header):

After (White Header):

[devin-ai-integration]

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