Fix panic opening the theme selector (#3499)

I noticed a panic when trying to open the theme selector. Removing `Arc`
from `cx.global::<Arc<ThemeRegistry>>()` did the job. Is that the right
way? I'm not sure. 😅

Release Notes:

- N/A
62bfa3b452Federico Dionisi committed on 12/5/2023, 2:28:48 PM· committed by GitHubparentsd433da120352c5
1 file changedLine totals unavailable