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/agent_ui/src/agent_diff.rs

@@ -1814,6 +1814,7 @@ mod tests {
     use serde_json::json;
     use settings::{Settings, SettingsStore};
     use std::{path::Path, rc::Rc};
+    use theme::ThemeSettings;
     use util::path;
 
     #[gpui::test]
@@ -1826,7 +1827,7 @@ mod tests {
             AgentSettings::register(cx);
             prompt_store::init(cx);
             workspace::init_settings(cx);
-            theme::init(theme::LoadThemes::JustBase, cx);
+            ThemeSettings::register(cx);
             EditorSettings::register(cx);
             language_model::init_settings(cx);
         });
@@ -1978,7 +1979,7 @@ mod tests {
             AgentSettings::register(cx);
             prompt_store::init(cx);
             workspace::init_settings(cx);
-            theme::init(theme::LoadThemes::JustBase, cx);
+            ThemeSettings::register(cx);
             EditorSettings::register(cx);
             language_model::init_settings(cx);
             workspace::register_project_item::<Editor>(cx);