Remove cx from ThemeSettings (#38836)

Before this change the active theme and icon theme were retrofitted onto
the ThemeSettings.

Now they're in their own new global (GlobalTheme::theme(cx) and
GlobalTheme::icon_theme(cx))

This lets us remove cx from the settings traits, and tidy up a few other
things along the way.

Release Notes:

- N/A

crates/agent/src/thread.rs

@@ -3276,7 +3276,6 @@ mod tests {
     use settings::{LanguageModelParameters, Settings, SettingsStore};
     use std::sync::Arc;
     use std::time::Duration;
-    use theme::ThemeSettings;
     use util::path;
     use workspace::Workspace;
 
@@ -5337,7 +5336,7 @@ fn main() {{
             thread_store::init(fs.clone(), cx);
             workspace::init_settings(cx);
             language_model::init_settings(cx);
-            ThemeSettings::register(cx);
+            theme::init(theme::LoadThemes::JustBase, cx);
             ToolRegistry::default_global(cx);
             assistant_tool::init(cx);