Header, navbar, and admin sidebar refactoring (minor fixes)

[davide-negretti]

References

• Additional fix for Header and navbar refactoring #2656
• The fix is already included in 7.x - Header, navbar, and admin sidebar refactoring #2858 (porting on DSpace 7.x)

Description

This PR fixes the color of breadcrumb links

Checklist

• My PR is small in size (e.g. less than 1,000 lines of code, not including comments & specs/tests), or I have provided reasons as to why that's not possible.
• My PR passes ESLint validation using yarn lint
• My PR doesn't introduce circular dependencies (verified via yarn check-circ-deps)
• My PR includes TypeDoc comments for all new (or modified) public methods and classes. It also includes TypeDoc for large or complex private methods.
• My PR passes all specs/tests and includes new/updated specs or tests based on the Code Testing Guide.
• If my PR includes new libraries/dependencies (in package.json), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.
• If my PR includes new features or configurations, I've provided basic technical documentation in the PR itself.
• If my PR fixes an issue ticket, I've linked them together.

[tdonohue]

👍 Thanks @davide-negretti ! This looks correct. Gave it a brief test to make sure it works and it does.