Fix default theme dark mode colors

[akirk]

Summary

• add light/dark color tokens for the default Friends frontend theme
• apply those tokens to the page shell, cards, forms, menus, subscriptions, and messages
• keep the existing light mode palette while honoring system dark mode

Testing

• composer check-cs
• git diff --check

Changelog

• Automatically create a changelog entry from the details below

Type

• Fixed

Message

Fix default frontend theme colors so the page follows system dark mode.

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