Substitute ui_charcoal theme deprecated by GitLab Theme (10.0) with ui_indigo

What does this MR do?

Substitutes the deprecated ui_charcoal (!9199 (merged)) with the new (10.0) default ui_indigo (!13619 (merged)).

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

To keep code consistence and to keep color consistence between https://gitlab.com/gitlab-org/gitlab-ce/issues and https://gitlab.com/users/sign_in.

Screenshots (if relevant)

before (taken from gitlab.com)	after

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• [n/a] Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [n/a] Has been reviewed by Backend
  • [n/a] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [n/a] Internationalization required/considered

What are the relevant issue numbers?

Closes #41744 (closed)