fix: support dark theme article card backgrounds

[saurabhhhcodes]

Closes #2128

Summary

• use a dark-safe placeholder background for article avatars
• use a theme-aware background for the bookmark action container
• keep existing light-theme colors unchanged

Validation

• git diff --check passed
• ESLint and TypeScript binaries were unavailable in the isolated checkout.

[github-actions]

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[github-actions]

❌ PR Validation Failed

This PR was marked as gssoc:invalid because: You are not the author or an assignee of the linked issue(s).

To resolve this, please ensure your PR description links an issue (e.g., Fixes #123) and that you are either the creator or an assignee of that issue.

[saurabhhhcodes]

Closing this PR because UltimateHealth validation marked the linked issue unassigned for this contributor (gssoc:invalid). The issue requires maintainer assignment before a valid GSSoC PR can be submitted.