[FIX] Dropdown visibility in the dark mode

[Rajitha-20050412]

Which issue does this PR close?

• Closes [Bug] Dropdown menu in the car billing system inconsistency in dark mode #995

Rationale for this change

The dropdown menu in the car billing page was not clearly visible in dark mode due to the lack of proper theme styling. This caused low contrast between the text and background, affecting readability and user experience. The fix ensures the dropdown is clearly visible and consistent with the overall dark theme design.

What changes are included in this PR?

• Added dark mode styling for the car brand dropdown (#carBrand) in the billing form.
• Updated background, text, and border colors for better contrast in dark mode.
• Added smooth transition effects between light and dark themes.
• Ensured dropdown options also adapt correctly in dark mode.

Are these changes tested?

• Verified that the dropdown displays properly in both light and dark modes.
• Tested the dark mode toggle to ensure smooth visual transitions.
• Confirmed consistent styling across browsers.

Are there any user-facing changes?

• Users will now see a properly visible and themed dropdown menu in dark mode on the car billing page.

[github-actions]

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• Ensure all file changes are correct.
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• Address any pending review comments if applicable.

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[Rajitha-20050412]

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