feat(navbar): update styles and improve UI

[youngbeom-shin]

What this PR does:

• Change hover text color for menu items
• Adjust active item font weight
• Update padding and margins for menu items
• Modify avatar sizes in dropdowns
• Refactor CSS for better consistency
• Enhance responsiveness for navbar items
• Update icon sizes for better alignment
• Improve text colors for better readability
  Which issue(s) this PR fixes:

Fixes # gitlab-724

Type of changes
Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Feature/Issue validation/testing:

Please describe the tests that you ran to verify your changes and relevant result summary.

• Test A

• Test B

• Logs

Special notes for your reviewer:

Checklist:

• I have added unit/e2e tests that prove your fix is effective or that this feature works.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• I have reviewed my own code and ensured that it follows the project's style guidelines.

Release note:

[zhendi]

LGTM