Switch arkose labs challenge to dark theme in LWTR experiment

What does this MR do and why?

Switch the Arkose Labs challenge on the trial_registrations/new page and subsequent identity verification page to dark theme for lightweight_trial_registration_redesign experiment candidate.

References

Screenshots or screen recordings

Control (original)	Candidate

How to set up and validate locally

Exact same steps as in !193360 (merged)

MR acceptance checklist

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