diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index aae8255..8f86319 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -166,6 +166,8 @@ pub struct CadApp {
     styled: bool,
     /// The theme last installed into egui (`None` until the first frame).
     shown_theme: Option<egui::Theme>,
+    /// Whether the last applied theme was handed to egui as the System preference.
+    shown_follows_system: bool,
     pub frame_ms: f64,
     pub quit_requested: bool,
     /// The app may close although drawings have unsaved changes (they were dealt with, or the
@@ -195,6 +197,7 @@ impl CadApp {
             frame_cap: None,
             styled: false,
             shown_theme: None,
+            shown_follows_system: false,
             frame_ms: 0.0,
             quit_requested: false,
             quit_confirmed: false,
@@ -359,9 +362,11 @@ impl CadApp {
         // browser report changes as input); the saved choice itself never changes here.
         let shown = self.ui.theme.resolve(ctx.system_theme());
         theme::set_active(shown);
-        if self.shown_theme != Some(shown) {
-            theme::apply(ctx, shown);
+        let follows_system = self.ui.theme == theme::ThemePref::System;
+        if self.shown_theme != Some(shown) || self.shown_follows_system != follows_system {
+            theme::apply(ctx, shown, follows_system);
             self.shown_theme = Some(shown);
+            self.shown_follows_system = follows_system;
         }
         self.drain_control(ctx);
         if !self.synthetic.is_empty() {
@@ -582,6 +587,22 @@ mod tests {
         assert_eq!(a.prefs_json(), r#"{"interfaceLanguage":"auto","saveFormat":"dxf","theme":"system"}"#);
     }
 
+    #[test]
+    fn system_theme_keeps_egui_on_the_system_preference() {
+        let mut a = app();
+        a.start("ui.theme.system");
+        let ctx = egui::Context::default();
+        frame(&mut a, &ctx, Some(egui::Theme::Light));
+        // A concrete Light would pin the native window appearance and hide later OS changes.
+        assert_eq!(ctx.options(|o| o.theme_preference), egui::ThemePreference::System);
+        a.start("ui.theme.dark");
+        frame(&mut a, &ctx, Some(egui::Theme::Light));
+        assert_eq!(ctx.options(|o| o.theme_preference), egui::ThemePreference::Dark);
+        a.start("ui.theme.system");
+        frame(&mut a, &ctx, Some(egui::Theme::Light));
+        assert_eq!(ctx.options(|o| o.theme_preference), egui::ThemePreference::System);
+    }
+
     #[test]
     fn default_save_format_persists_and_names_new_drawings() {
         let mut a = app();
diff --git a/crates/ui-egui/src/theme.rs b/crates/ui-egui/src/theme.rs
index 0c299bd..00c541c 100644
--- a/crates/ui-egui/src/theme.rs
+++ b/crates/ui-egui/src/theme.rs
@@ -284,12 +284,15 @@ fn visuals(theme: Theme) -> Visuals {
 }
 
 /// Install the styles of both themes and show `theme` (egui's own widgets, popups and menus
-/// follow it; our own painting follows [`Tokens::get`]).
-pub fn apply(ctx: &egui::Context, theme: Theme) {
+/// follow it; our own painting follows [`Tokens::get`]). `follow_system` is true for the
+/// [`ThemePref::System`] choice, whose `theme` was resolved from the OS report.
+pub fn apply(ctx: &egui::Context, theme: Theme, follow_system: bool) {
     set_active(theme);
     ctx.set_visuals_of(Theme::Dark, visuals(Theme::Dark));
     ctx.set_visuals_of(Theme::Light, visuals(Theme::Light));
-    ctx.set_theme(theme);
+    // A System choice keeps egui's own System preference: a concrete Light/Dark would pin the native
+    // window appearance (winit then stops reporting OS changes) and the app would never follow again.
+    ctx.set_theme(if follow_system { egui::ThemePreference::System } else { egui::ThemePreference::from(theme) });
     ctx.all_styles_mut(|s| {
         s.spacing.item_spacing = egui::vec2(6.0, 4.0);
         s.spacing.button_padding = egui::vec2(6.0, 3.0);