In the options ui, show only the currently selected theme

* Closes https://github.com/emilk/egui/pull/5101

crates/egui/src/memory/mod.rs

@@ -362,6 +362,8 @@ impl Options {
 impl Options {
     /// Show the options in the ui.
     pub fn ui(&mut self, ui: &mut crate::Ui) {
+        let theme = self.theme();
+
         let Self {
             dark_style, // covered above
             light_style,
@@ -383,6 +385,7 @@ impl Options {
             reduce_texture_memory,
         } = self;
 
+        use crate::containers::CollapsingHeader;
         use crate::Widget as _;
 
         CollapsingHeader::new("⚙ Options")
@@ -408,18 +411,16 @@ impl Options {
                 ui.checkbox(reduce_texture_memory, "Reduce texture memory");
             });
 
-        use crate::containers::CollapsingHeader;
         CollapsingHeader::new("🎑 Style")
             .default_open(true)
             .show(ui, |ui| {
                 theme_preference.radio_buttons(ui);
 
-                CollapsingHeader::new("Dark")
-                    .default_open(true)
-                    .show(ui, |ui| std::sync::Arc::make_mut(dark_style).ui(ui));
-                CollapsingHeader::new("Light")
-                    .default_open(true)
-                    .show(ui, |ui| std::sync::Arc::make_mut(light_style).ui(ui));
+                std::sync::Arc::make_mut(match theme {
+                    Theme::Dark => dark_style,
+                    Theme::Light => light_style,
+                })
+                .ui(ui);
             });
 
         CollapsingHeader::new("✒ Painting")