Reland "Remove cx from ThemeSettings" (#39720)

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

Closes #ISSUE

Release Notes:

- N/A

crates/git_ui/src/git_panel_settings.rs

@@ -43,7 +43,7 @@ impl ScrollbarVisibility for GitPanelSettings {
 }
 
 impl Settings for GitPanelSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut ui::App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let git_panel = content.git_panel.clone().unwrap();
         Self {
             button: git_panel.button.unwrap(),