Reland "Remove cx from ThemeSettings" (#39720)

- **Reapply "Remove cx from ThemeSettings (#38836)" (#39691)**
- **Fix theme loading races**

Closes #ISSUE

Release Notes:

- N/A

crates/settings_profile_selector/src/settings_profile_selector.rs

@@ -302,7 +302,6 @@ mod tests {
             cx.set_global(settings_store);
             settings::init(cx);
             theme::init(theme::LoadThemes::JustBase, cx);
-            ThemeSettings::register(cx);
             client::init_settings(cx);
             language::init(cx);
             super::init(cx);