[FIX] Logo visibility in the dark mode

[Rajitha-20050412]

Which issue does this PR close?

• Closes [Bug] Logo not properly visible in dark mode #991

Rationale for this change

The navbar and footer logos were not visible in dark mode due to the existing CSS filter, which made the colored logo disappear. This fix ensures the logos are visible and consistent in both light and dark themes.

What changes are included in this PR?

• Added dark-mode versions of the navbar and footer logos.
• Updated CSS to switch logos when .dark-theme is active.
• Removed the previous brightness(0) invert(1) filter from dark mode styling.

Are these changes tested?

• Verified that the navbar and footer logos display correctly in both light and dark modes.
• Tested smooth switching when toggling dark mode.

Are there any user-facing changes?

• Users will now see the correct logos in both light and dark modes instead of the alt text.

[github-actions]

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• Address any pending review comments if applicable.

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[Rajitha-20050412]

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