fix(NcGuestContent): migrate style assignment fully to CSS

[Antreesy]

☑️ Resolves

• Suppress mentioned TypeError in production code
  • There is nothing relevant for end-user to see, and should be handled by developer
• Assisted-by: ClaudeCode:claude-sonnet-4-6

🖼️ Screenshots

🚧 Tasks

• Check upstream pages that render it (maintenance / upgrade / pass protection)?
• Are added styles required at all?

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 2️⃣ Backport to stable8 for maintained Vue 2 version or not applicable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.06%. Comparing base (f54323d) to head (94fe6fe).
⚠️ Report is 31 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8677   +/-   ##
=======================================
  Coverage   54.06%   54.06%           
=======================================
  Files         106      106           
  Lines        3479     3479           
  Branches     1008     1008           
=======================================
  Hits         1881     1881           
  Misses       1354     1354           
  Partials      244      244           

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[mejo-]

Code changes look sensible to me.

[Antreesy]

/backport to stable8