[FIX] Fixed navbar styling in dark mode for buy car page

[Rajitha-20050412]

Which issue does this PR close?
Closes #987

Rationale for this change
The navigation bar on the Buy Used Cars page had inconsistent styling when switching to dark mode. The background color, text contrast, and hover effects did not align with the overall dark theme used across other pages, affecting visual uniformity and user experience.
This fix ensures consistency in dark mode styling throughout the site.

What changes are included in this PR?
Fixed the navbar background color to match the global dark theme.
Updated text and icon colors for proper visibility in dark mode.
Adjusted hover and active states for better contrast.
Ensured responsive behavior is consistent across light and dark themes.
Cleaned up redundant CSS rules for dark mode styling.

Are these changes tested?
Yes, the changes were tested manually by:
Switching between light and dark modes to verify consistency.
Checking the navbar across different screen sizes (mobile, tablet, desktop).
Verifying that navigation links and dropdowns remain functional in both modes.

Are there any user-facing changes?
Yes. Users will now experience:
A consistent and visually balanced navbar when using dark mode on the Buy Used Cars page.
Improved readability and contrast for all navbar elements.
Seamless theme switching without color mismatches.

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