theme_selector: Mark the active theme with a check icon (#58404)

While browsing themes in theme selector, the currently active theme will
always be marked with a check icon so you can always have a reference of
where you started from.

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)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Here's a preview: 
<img width="950" height="806" alt="Screenshot 2026-06-03 at 13 09 46"
src="https://github.com/user-attachments/assets/2081d9ec-b56d-4542-be58-e59e3569bc7a"
/>
<img width="958" height="818" alt="Screenshot 2026-06-03 at 13 09 52"
src="https://github.com/user-attachments/assets/134d933f-c9de-4b2c-80f5-ed5be4fb2844"
/>


Release Notes:

- Improved the theme and icon selectors to display a check next to the
active theme

---------

Co-authored-by: dino <dinojoaocosta@gmail.com>

crates/theme_selector/src/icon_theme_selector.rs

@@ -299,6 +299,7 @@ impl PickerDelegate for IconThemeSelectorDelegate {
         _cx: &mut Context<Picker<Self>>,
     ) -> Option<Self::ListItem> {
         let theme_match = &self.matches.get(ix)?;
+        let is_original_theme = theme_match.string.as_str() == self.original_theme.0.as_ref();
 
         Some(
             ListItem::new(ix)
@@ -308,7 +309,10 @@ impl PickerDelegate for IconThemeSelectorDelegate {
                 .child(HighlightedLabel::new(
                     theme_match.string.clone(),
                     theme_match.positions.clone(),
-                )),
+                ))
+                .when(is_original_theme, |this| {
+                    this.end_slot(Icon::new(IconName::Check).color(Color::Muted))
+                }),
         )
     }
 

crates/theme_selector/src/theme_selector.rs

@@ -134,6 +134,14 @@ struct ThemeSelectorDelegate {
     original_theme_settings: ThemeSettings,
     /// The current system appearance.
     original_system_appearance: Appearance,
+    /// The id of the original theme in the list of themes.
+    /// Using `Option<usize>` instead of `usize` because it's possible that the
+    /// original theme is not present in the list of themes when it is first
+    /// built, depending on the provided `themes_filter`. For example, when a
+    /// theme is installed, the `themes_filter` is set to the new theme names
+    /// and, if we used `unwrap_or(0)` as a fallback, the first theme in the
+    /// list would be shown as "active".
+    original_theme_id: Option<usize>,
     /// The currently selected new theme.
     new_theme: Arc<Theme>,
     selection_completed: bool,
@@ -174,10 +182,15 @@ impl ThemeSelectorDelegate {
                 .then(a.name.cmp(&b.name))
         });
 
+        let original_theme_id = themes
+            .iter()
+            .position(|meta| meta.name == original_theme.name);
+
         let matches: Vec<StringMatch> = themes
             .iter()
-            .map(|meta| StringMatch {
-                candidate_id: 0,
+            .enumerate()
+            .map(|(id, meta)| StringMatch {
+                candidate_id: id,
                 score: 0.0,
                 positions: Default::default(),
                 string: meta.name.to_string(),
@@ -196,6 +209,7 @@ impl ThemeSelectorDelegate {
             matches,
             original_theme_settings,
             original_system_appearance,
+            original_theme_id,
             new_theme: original_theme, // Start with the original theme.
             selected_index,
             selection_completed: false,
@@ -204,6 +218,13 @@ impl ThemeSelectorDelegate {
         }
     }
 
+    fn is_original_theme(&self, index: usize) -> bool {
+        self.matches
+            .get(index)
+            .zip(self.original_theme_id)
+            .is_some_and(|(mat, original_theme_id)| mat.candidate_id == original_theme_id)
+    }
+
     fn show_selected_theme(
         &mut self,
         cx: &mut Context<Picker<ThemeSelectorDelegate>>,
@@ -487,6 +508,7 @@ impl PickerDelegate for ThemeSelectorDelegate {
         _cx: &mut Context<Picker<Self>>,
     ) -> Option<Self::ListItem> {
         let theme_match = &self.matches.get(ix)?;
+        let is_original_theme = self.is_original_theme(ix);
 
         Some(
             ListItem::new(ix)
@@ -496,7 +518,10 @@ impl PickerDelegate for ThemeSelectorDelegate {
                 .child(HighlightedLabel::new(
                     theme_match.string.clone(),
                     theme_match.positions.clone(),
-                )),
+                ))
+                .when(is_original_theme, |this| {
+                    this.end_slot(Icon::new(IconName::Check).color(Color::Muted))
+                }),
         )
     }