Improve contrast of Links in Dark Mode

Summary:
In Dark Mode, for links, use lighter color, to obtain higher contrast to dark background.

| Before | After |
|------------|-------------|
| {F5102768} | {F5102787} |

Explanation:

{F416016}

Ref T15056

Test Plan:
- Purge all Phorge caches with ./bin/cache purge --all
- Sign in
- Go to user's Settings > Display Preferences and select the Accessibility (user interface) "Dark Mode".
- Check Dashboard links
- Check People lists
- Check Maniphest lists
- Check Diffusion lists
- Check Feeds, etc.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: grembo, deric, 20after4, Iniquity, aklapper, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Tags: #accessibility, #ux

Maniphest Tasks: T15056

Differential Revision: https://we.phorge.it/D25491