fix: groupmentions have poor contrast on some backgrounds

[imorland]

First noticed on discuss today, some group colors present poor text contrast.

For the record, I would not have been able to complete this without the help of @davwheat - thank you David!

Changes proposed in this pull request:
Utilize the yiq helper recently introduced to add either a --light or --dark class

Reviewers should focus on:

Screenshot
Before:

After:

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[ornanovitch]

I'm so glad this yiq helper already helps! ♥️