settings_ui: Fix switching icon themes from "dynamic" to "static" always selecting the dark theme (#54647)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #54646 

Release Notes:

- Fixed settings behavior where switching Icon Theme from "Dynamic"
(System mode) to "Static" now selects the icon theme that matches the
current OS appearance (light or dark), instead of always defaulting to
the dark variant.

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/settings_ui/src/page_data.rs

@@ -622,7 +622,7 @@ fn appearance_page() -> SettingsPage {
                                     .as_ref()?
                                     .discriminant() as usize])
                         },
-                        write: |settings_content, value, _| {
+                        write: |settings_content, value, app| {
                             let Some(value) = value else {
                                 settings_content.theme.icon_theme = None;
                                 return;
@@ -638,7 +638,13 @@ fn appearance_page() -> SettingsPage {
                                             match mode {
                                                 theme_settings::ThemeAppearanceMode::Light => light.clone(),
                                                 theme_settings::ThemeAppearanceMode::Dark => dark.clone(),
-                                                theme_settings::ThemeAppearanceMode::System => dark.clone(), // no cx, can't determine correct choice
+                                                theme_settings::ThemeAppearanceMode::System => {
+                                                    if SystemAppearance::global(app).is_light() {
+                                                        light.clone()
+                                                    } else {
+                                                        dark.clone()
+                                                    }
+                                                }
                                             }
                                         },
                                     };