Revert "Remove cx from ThemeSettings (#38836)" (#39691)

This reverts commit a2a7bd139a.

This caused themes to not load correctly on startup, you needed to edit
your settings.

Release Notes:

- N/A

crates/auto_update/src/auto_update.rs

@@ -127,7 +127,7 @@ struct AutoUpdateSetting(bool);
 ///
 /// Default: true
 impl Settings for AutoUpdateSetting {
-    fn from_settings(content: &settings::SettingsContent) -> Self {
+    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
         Self(content.auto_update.unwrap())
     }
 }