fix: multiple issues in link styles

[huyenltnguyen]

Checklist:

• [x] I have read freeCodeCamp's contribution guidelines.
• [x] My pull request has a descriptive title (not a vague title like Update index.md)

This PR fixes the following issues:

• Text color of active sidebar items not having sufficient contrast
• Active link having a blue underline (this apparently came from a Discourse change. Having the underline isn't a big deal, but I think it's redundant)
• Some link icons are in blue instead of using the link text color

Screenshots

| Issue | Before | After | | --- | --- | --- | | Sidebar item | | | | Link underline | | | | Link icon | | |