diff --git a/crates/ui-egui/src/backstage.rs b/crates/ui-egui/src/backstage.rs
index 1ff5ae7..2c5b4bc 100644
--- a/crates/ui-egui/src/backstage.rs
+++ b/crates/ui-egui/src/backstage.rs
@@ -279,10 +279,7 @@ fn options_page(app: &mut WordApp, ui: &mut Ui) {
             app.session.author = n;
         }
     });
-    let mut dark = app.ui.dark;
-    if ui.checkbox(&mut dark, tl!("Dark mode")).changed() {
-        let _ = app.run("ui.dark", json!({"value": dark}));
-    }
+    theme_picker(app, ui);
     ui.checkbox(&mut app.autosave, tl!("AutoSave documents you have saved in WordCraft"));
     let mut dark_page = app.session.view.dark_mode;
     if ui
@@ -308,6 +305,21 @@ fn options_page(app: &mut WordApp, ui: &mut Ui) {
     ui.label(tl!("Every command is available to scripts and AI agents: run `wordcraft-cli mcp` for an MCP server, or start the app with `--control <port>` for the JSON control channel."));
 }
 
+/// File ▸ Options ▸ Interface theme: Light, Dark, or follow the system's appearance (#115).
+fn theme_picker(app: &mut WordApp, ui: &mut Ui) {
+    use crate::theme::Appearance;
+    ui.horizontal(|ui| {
+        ui.label(tl!("Interface theme:"));
+        egui::ComboBox::from_id_salt("interface_theme").selected_text(tl!(app.ui.theme.label())).width(220.0).show_ui(ui, |ui| {
+            for a in Appearance::ALL {
+                if ui.selectable_label(app.ui.theme == a, tl!(a.label())).clicked() {
+                    let _ = app.run("ui.theme", json!({"value": a.code()}));
+                }
+            }
+        });
+    });
+}
+
 /// File ▸ Options ▸ Interface language: follow the system (the default) or pick one (#8).
 fn language_picker(app: &mut WordApp, ui: &mut Ui) {
     use crate::i18n::{AUTO, Lang};
diff --git a/crates/ui-egui/src/i18n/es.tsv b/crates/ui-egui/src/i18n/es.tsv
index 4858ea7..fdb5bd8 100644
--- a/crates/ui-egui/src/i18n/es.tsv
+++ b/crates/ui-egui/src/i18n/es.tsv
@@ -164,7 +164,6 @@
 	Customize Quick Access Toolbar	Personalizar la barra de herramientas de acceso rápido
 	Cut	Cortar
 	Dark Mode	Modo oscuro
-	Dark mode	Modo oscuro
 	Dashed	Discontinua
 	Data	Datos
 	Date & Time	Fecha y hora
@@ -893,3 +892,9 @@
 	Opening print dialog…	Abriendo el cuadro de diálogo de impresión…
 	Print failed: {error}	Error al imprimir: {error}
 	Couldn't open {path}: {error}	No se pudo abrir {path}: {error}
+	Interface theme:	Tema de la interfaz:
+	Interface\nTheme	Tema de la\ninterfaz
+	Interface Theme	Tema de la interfaz
+	Light	Claro
+	Dark	Oscuro
+	Use system setting	Usar la configuración del sistema
diff --git a/crates/ui-egui/src/i18n/ja.tsv b/crates/ui-egui/src/i18n/ja.tsv
index dc5999f..f486551 100644
--- a/crates/ui-egui/src/i18n/ja.tsv
+++ b/crates/ui-egui/src/i18n/ja.tsv
@@ -157,7 +157,6 @@
 	Customize Quick Access Toolbar	クイック アクセス ツール バーのユーザー設定
 	Cut	切り取り
 	Dark Mode	ダーク モード
-	Dark mode	ダーク モード
 	Dashed	破線
 	Data	データ
 	Date & Time	日付と時刻
@@ -876,3 +875,9 @@
 	Opening print dialog…	印刷ダイアログを開いています…
 	Print failed: {error}	印刷できませんでした: {error}
 	Couldn't open {path}: {error}	{path} を開けませんでした: {error}
+	Interface theme:	表示テーマ:
+	Interface\nTheme	表示テーマ
+	Interface Theme	表示テーマ
+	Light	ライト
+	Dark	ダーク
+	Use system setting	システム設定を使用
diff --git a/crates/ui-egui/src/i18n/pt-br.tsv b/crates/ui-egui/src/i18n/pt-br.tsv
index c6c6bea..9dfc016 100644
--- a/crates/ui-egui/src/i18n/pt-br.tsv
+++ b/crates/ui-egui/src/i18n/pt-br.tsv
@@ -161,7 +161,6 @@
 	Customize Quick Access Toolbar	Personalizar barra de acesso rápido
 	Cut	Recortar
 	Dark Mode	Modo escuro
-	Dark mode	Modo escuro
 	Dashed	Tracejada
 	Data	Dados
 	Date & Time	Data e hora
@@ -888,3 +887,9 @@
 	Opening print dialog…	Abrindo a caixa de diálogo de impressão…
 	Print failed: {error}	Falha na impressão: {error}
 	Couldn't open {path}: {error}	Não foi possível abrir {path}: {error}
+	Interface theme:	Tema da interface:
+	Interface\nTheme	Tema da\ninterface
+	Interface Theme	Tema da interface
+	Light	Claro
+	Dark	Escuro
+	Use system setting	Usar a configuração do sistema
diff --git a/crates/ui-egui/src/i18n/uk.tsv b/crates/ui-egui/src/i18n/uk.tsv
index f356a63..25b9a4d 100644
--- a/crates/ui-egui/src/i18n/uk.tsv
+++ b/crates/ui-egui/src/i18n/uk.tsv
@@ -161,7 +161,6 @@
 	Customize Quick Access Toolbar	Налаштувати панель швидкого доступу
 	Cut	Вирізати
 	Dark Mode	Темний режим
-	Dark mode	Темний режим
 	Dashed	Штрихова
 	Data	Дані
 	Date & Time	Дата й час
@@ -888,3 +887,9 @@
 	Opening print dialog…	Відкриття діалогу друку…
 	Print failed: {error}	Не вдалося надрукувати: {error}
 	Couldn't open {path}: {error}	Не вдалося відкрити {path}: {error}
+	Interface theme:	Тема інтерфейсу:
+	Interface\nTheme	Тема\nінтерфейсу
+	Interface Theme	Тема інтерфейсу
+	Light	Світла
+	Dark	Темна
+	Use system setting	Як у системі
diff --git a/crates/ui-egui/src/i18n/zh-hans.tsv b/crates/ui-egui/src/i18n/zh-hans.tsv
index 01154be..9e4856d 100644
--- a/crates/ui-egui/src/i18n/zh-hans.tsv
+++ b/crates/ui-egui/src/i18n/zh-hans.tsv
@@ -168,7 +168,6 @@
 	Customize Quick Access Toolbar	自定义快速访问工具栏
 	Cut	剪切
 	Dark Mode	深色模式
-	Dark mode	深色模式
 	Dashed	虚线
 	Data	数据
 	Date & Time	日期和时间
@@ -887,3 +886,9 @@
 	Opening print dialog…	正在打开打印对话框…
 	Print failed: {error}	打印失败：{error}
 	Couldn't open {path}: {error}	无法打开 {path}：{error}
+	Interface theme:	界面主题：
+	Interface\nTheme	界面主题
+	Interface Theme	界面主题
+	Light	浅色
+	Dark	深色
+	Use system setting	使用系统设置
diff --git a/crates/ui-egui/src/i18n/zh-hant.tsv b/crates/ui-egui/src/i18n/zh-hant.tsv
index a1e6e2b..9adadea 100644
--- a/crates/ui-egui/src/i18n/zh-hant.tsv
+++ b/crates/ui-egui/src/i18n/zh-hant.tsv
@@ -158,7 +158,6 @@
 	Customize Quick Access Toolbar	自訂快速存取工具列
 	Cut	剪下
 	Dark Mode	深色模式
-	Dark mode	深色模式
 	Dashed	虛線
 	Data	資料
 	Date & Time	日期及時間
@@ -877,3 +876,9 @@
 	Opening print dialog…	正在開啟列印對話方塊…
 	Print failed: {error}	列印失敗：{error}
 	Couldn't open {path}: {error}	無法開啟 {path}：{error}
+	Interface theme:	介面主題：
+	Interface\nTheme	介面主題
+	Interface Theme	介面主題
+	Light	淺色
+	Dark	深色
+	Use system setting	使用系統設定
diff --git a/crates/ui-egui/src/icons.rs b/crates/ui-egui/src/icons.rs
index 8ff68a9..2491375 100644
--- a/crates/ui-egui/src/icons.rs
+++ b/crates/ui-egui/src/icons.rs
@@ -874,6 +874,12 @@ pub fn paint(p: &Painter, r: Rect, name: &str, c: Color32, accent: Color32) {
             pen.fcircle(10.0, 10.0, 7.0, c);
             pen.fcircle(13.0, 8.0, 6.0, Color32::from_rgba_unmultiplied(255, 255, 255, 230));
         }
+        "interfaceTheme" => {
+            // A window, its right half dark: light, dark or both (follow the system).
+            pen.frect(10.0, 3.5, 17.5, 16.5, c);
+            pen.rect(2.5, 3.5, 17.5, 16.5, c);
+            pen.line_c(&[(2.5, 6.5), (10.0, 6.5)], a);
+        }
         "newWindow" => {
             pen.rect(2.5, 5.0, 13.0, 15.0, c);
             pen.rect(7.0, 2.5, 17.5, 12.5, a);
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index f882858..d9a44fb 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -76,7 +76,11 @@ pub struct UiState {
     pub backstage_page: String,
     pub ribbon_collapsed: bool,
     pub recent: Vec<String>,
-    pub dark: bool,
+    /// Interface theme: light, dark, or follow the OS appearance (#115).
+    pub theme: theme::Appearance,
+    /// The dark-mode switch `ui.json` held before `theme` (#115): read once to migrate, never written.
+    #[serde(rename = "dark", skip_serializing)]
+    pub(crate) legacy_dark: Option<bool>,
     pub nav_tab: String,
     pub show_discord: bool,
     /// User name (File › Options) for comments and tracked changes; empty keeps the default.
@@ -105,7 +109,8 @@ impl Default for UiState {
             backstage_page: "home".into(),
             ribbon_collapsed: false,
             recent: Vec::new(),
-            dark: false,
+            theme: theme::Appearance::default(),
+            legacy_dark: None,
             nav_tab: "headings".into(),
             show_discord: true,
             author: String::new(),
@@ -244,6 +249,10 @@ impl WordApp {
     pub fn apply_prefs(&mut self, ui: UiState) {
         self.ui = ui;
         self.ui.backstage = false;
+        // A `ui.json` from before the theme setting: its dark-mode switch picks Dark or Light.
+        if let Some(dark) = self.ui.legacy_dark.take() {
+            self.ui.theme = if dark { theme::Appearance::Dark } else { theme::Appearance::Light };
+        }
         // The session owns the name from here on; `prefs` copies it back when saving.
         let author = std::mem::take(&mut self.ui.author);
         if !author.trim().is_empty() {
@@ -426,6 +435,11 @@ impl WordApp {
         }
     }
 
+    /// Whether the interface theme setting currently resolves to dark (`System` asks the OS).
+    pub fn ui_is_dark(&self) -> bool {
+        self.ui.theme.is_dark(self.ctx.as_ref().and_then(egui::Context::system_theme))
+    }
+
     /// Commands that live in the UI layer.
     fn ui_command(&mut self, id: &str, p: &Value) -> Option<Result<Value, String>> {
         let s = |k: &str| p.get(k).and_then(Value::as_str);
@@ -468,8 +482,20 @@ impl WordApp {
                 json!({"collapsed": self.ui.ribbon_collapsed})
             }
             "ui.dark" => {
-                self.ui.dark = p.get("value").and_then(Value::as_bool).unwrap_or(!self.ui.dark);
-                json!({"dark": self.ui.dark})
+                // A manual Light/Dark switch (toggles what is shown when no value is given).
+                let dark = p.get("value").and_then(Value::as_bool).unwrap_or(!self.ui_is_dark());
+                self.ui.theme = if dark { theme::Appearance::Dark } else { theme::Appearance::Light };
+                json!({"dark": dark, "theme": self.ui.theme.code()})
+            }
+            "ui.theme" => {
+                // `light`, `dark` or `system` (follow the OS appearance); no value reads it.
+                if let Some(v) = s("value") {
+                    match theme::Appearance::from_code(v) {
+                        Some(a) => self.ui.theme = a,
+                        None => return Some(Err(format!("unknown theme {v:?}; use light, dark or system"))),
+                    }
+                }
+                json!({"theme": self.ui.theme.code(), "dark": self.ui_is_dark()})
             }
             "ui.language" => {
                 // `auto` (follow the system) or a language code; anything else is an error.
@@ -631,7 +657,9 @@ impl WordApp {
             ctx.options_mut(|o| o.zoom_with_keyboard = false);
             self.styled = true;
         }
-        let dark = self.ui.dark || self.session.view.dark_mode;
+        // Re-checked every frame, so `System` follows an OS appearance change live (egui reports it
+        // and repaints).
+        let dark = self.ui.theme.is_dark(ctx.system_theme()) || self.session.view.dark_mode;
         if self.applied_dark != Some(dark) {
             theme::apply(ctx, &if dark { theme::Tokens::dark() } else { theme::Tokens::light() });
             self.applied_dark = Some(dark);
@@ -1710,10 +1738,48 @@ mod tests {
         let default = a.session.author.clone();
         a.apply_prefs(serde_json::from_str(r#"{"dark": true, "backstage": true}"#).unwrap());
         assert_eq!(a.session.author, default);
-        assert!(a.ui.dark);
+        assert_eq!(a.ui.theme, theme::Appearance::Dark);
         assert!(!a.ui.backstage);
     }
 
+    /// #115: the old dark-mode switch migrates to Light/Dark; a saved System stays System, and the
+    /// old key is never written back.
+    #[test]
+    fn interface_theme_migrates_and_survives_a_restart() {
+        use theme::Appearance;
+        for (saved, want) in [(r#"{"dark": false}"#, Appearance::Light), (r#"{"dark": true}"#, Appearance::Dark), ("{}", Appearance::Light)] {
+            let mut a = app();
+            a.apply_prefs(serde_json::from_str(saved).unwrap());
+            assert_eq!(a.ui.theme, want, "{saved}");
+        }
+        let mut a = app();
+        a.run("ui.theme", json!({"value": "System"})).unwrap();
+        let saved = serde_json::to_string(&a.prefs()).unwrap();
+        assert!(saved.contains(r#""theme":"system""#) && !saved.contains(r#""dark":"#), "{saved}");
+        let mut b = app();
+        b.apply_prefs(serde_json::from_str(&saved).unwrap());
+        assert_eq!(b.ui.theme, Appearance::System);
+        // An unknown saved value keeps the rest of the preferences.
+        let odd: UiState = serde_json::from_str(r#"{"theme": "sepia", "tab": "Insert"}"#).unwrap();
+        assert_eq!((odd.theme, odd.tab.as_str()), (Appearance::Light, "Insert"));
+    }
+
+    /// #115: System follows the OS appearance (light when unknown); the manual choices ignore it.
+    #[test]
+    fn interface_theme_resolves_against_the_os_appearance() {
+        use egui::Theme::{Dark, Light};
+        use theme::Appearance;
+        let cases = [(Appearance::System, None, false), (Appearance::System, Some(Light), false), (Appearance::System, Some(Dark), true)];
+        for (setting, os, dark) in cases.into_iter().chain([(Appearance::Light, Some(Dark), false), (Appearance::Dark, Some(Light), true)]) {
+            assert_eq!(setting.is_dark(os), dark, "{setting:?} with the OS at {os:?}");
+        }
+        let mut a = app();
+        assert!(a.run("ui.theme", json!({"value": "purple"})).is_err());
+        assert_eq!(a.run("ui.theme", json!({"value": "dark"})).unwrap()["theme"], "dark");
+        assert_eq!(a.run("ui.dark", json!({})).unwrap()["theme"], "light", "ui.dark toggles the manual choice");
+        assert_eq!(a.run("ui.theme", json!({})).unwrap()["theme"], "light");
+    }
+
     #[test]
     fn word_count_setting_survives_a_restart() {
         let mut a = app();
@@ -1729,7 +1795,7 @@ mod tests {
     fn prefs_without_editing_keep_its_defaults() {
         let mut a = app();
         a.apply_prefs(serde_json::from_str(r#"{"tab": "Insert", "dark": true}"#).unwrap());
-        assert_eq!((a.ui.tab.as_str(), a.ui.dark), ("Insert", true));
+        assert_eq!((a.ui.tab.as_str(), a.ui.theme), ("Insert", theme::Appearance::Dark));
         assert!(a.session.prefs.count_notes, "text boxes and notes count by default");
     }
 
diff --git a/crates/ui-egui/src/ribbon.rs b/crates/ui-egui/src/ribbon.rs
index fcf9ded..7e592df 100644
--- a/crates/ui-egui/src/ribbon.rs
+++ b/crates/ui-egui/src/ribbon.rs
@@ -1015,6 +1015,14 @@ fn view(app: &mut WordApp, ui: &mut Ui) {
     });
     group(ui, "Dark Mode", None, app, |ui, app| {
         big(ui, app, "darkMode", "Switch\nModes", "view.darkMode", json!({}), false);
+        menu_button(ui, app, "interfaceTheme", Some("Interface\nTheme"), "Interface Theme", true, |ui, app| {
+            for a in crate::theme::Appearance::ALL {
+                if ui.add(egui::Button::selectable(app.ui.theme == a, tl!(a.label())).min_size(vec2(200.0, 0.0))).clicked() {
+                    let _ = app.run("ui.theme", json!({"value": a.code()}));
+                    ui.close();
+                }
+            }
+        });
     });
     group(ui, "Window", None, app, |ui, app| {
         big(ui, app, "newWindow", "New\nWindow", "view.newWindow", json!({}), false);
diff --git a/crates/ui-egui/src/theme.rs b/crates/ui-egui/src/theme.rs
index 52d5ba6..6bde325 100644
--- a/crates/ui-egui/src/theme.rs
+++ b/crates/ui-egui/src/theme.rs
@@ -8,6 +8,67 @@ use egui::{Align2, Color32, CornerRadius, FontData, FontDefinitions, FontFamily,
 pub const APP_COLOR: Color32 = Color32::from_rgb(0x3B, 0x5B, 0xDB);
 pub const APP_INK: Color32 = Color32::from_rgb(0x2B, 0x47, 0xB5);
 
+/// The interface theme setting (File › Options › General, View › Dark Mode): a fixed light or
+/// dark palette, or `System`, which follows the OS light/dark appearance live (#115).
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+#[serde(from = "String", into = "String")]
+pub enum Appearance {
+    #[default]
+    Light,
+    Dark,
+    System,
+}
+
+impl Appearance {
+    /// In menu order.
+    pub const ALL: [Appearance; 3] = [Appearance::System, Appearance::Light, Appearance::Dark];
+
+    /// The value used in `ui.json` and by the `ui.theme` command.
+    pub fn code(self) -> &'static str {
+        match self {
+            Appearance::Light => "light",
+            Appearance::Dark => "dark",
+            Appearance::System => "system",
+        }
+    }
+
+    pub fn from_code(code: &str) -> Option<Appearance> {
+        Appearance::ALL.into_iter().find(|a| a.code().eq_ignore_ascii_case(code.trim()))
+    }
+
+    /// The English label (translated with `tl!` where shown).
+    pub fn label(self) -> &'static str {
+        match self {
+            Appearance::Light => "Light",
+            Appearance::Dark => "Dark",
+            Appearance::System => "Use system setting",
+        }
+    }
+
+    /// Whether the interface is dark, given the OS appearance egui reports (`Context::system_theme`).
+    /// `System` falls back to light when the OS reports none.
+    pub fn is_dark(self, system: Option<egui::Theme>) -> bool {
+        match self {
+            Appearance::Light => false,
+            Appearance::Dark => true,
+            Appearance::System => system == Some(egui::Theme::Dark),
+        }
+    }
+}
+
+/// An unknown saved value reads as the default rather than discarding the whole `ui.json`.
+impl From<String> for Appearance {
+    fn from(s: String) -> Self {
+        Appearance::from_code(&s).unwrap_or_default()
+    }
+}
+
+impl From<Appearance> for String {
+    fn from(a: Appearance) -> Self {
+        a.code().to_string()
+    }
+}
+
 /// Every colour the UI uses; widgets never hard-code colours.
 #[derive(Clone, Copy, Debug)]
 pub struct Tokens {
@@ -301,6 +362,9 @@ pub fn apply(ctx: &egui::Context, t: &Tokens) {
     v.widgets.open.weak_bg_fill = t.pressed;
     v.popup_shadow = egui::Shadow { offset: [0, 4], blur: 16, spread: 0, color: Color32::from_black_alpha(if t.dark { 120 } else { 40 }) };
     v.window_shadow = egui::Shadow { offset: [0, 8], blur: 28, spread: 0, color: Color32::from_black_alpha(if t.dark { 140 } else { 50 }) };
+    // Pin egui's own theme to ours: left on its default (follow the OS), egui would swap to its
+    // other, unstyled light/dark style the moment the OS appearance changed.
+    ctx.set_theme(if t.dark { egui::Theme::Dark } else { egui::Theme::Light });
     ctx.set_visuals(v);
     ctx.global_style_mut(|s| {
         s.spacing.item_spacing = egui::vec2(6.0, 4.0);
diff --git a/docs/control-protocol.md b/docs/control-protocol.md
index 431224f..27aea7a 100644
--- a/docs/control-protocol.md
+++ b/docs/control-protocol.md
@@ -50,7 +50,7 @@ and set `WORDCRAFT_CONTROL_KEY` to that app's key.
 
 | Method | Params | Result |
 |---|---|---|
-| `engine.execute` | `command`, `params` | the command's result (any id from `engine.commands`, including UI commands `ui.tab`, `ui.dialog`, `ui.backstage`, `ui.zotero.*` (see `docs/zotero.md`), `ui.language` (`{"value": "auto"|"en"|"zh-hans"|"zh-hant"|"ja"|"pt-br"|"es"|"uk"}`)…) |
+| `engine.execute` | `command`, `params` | the command's result (any id from `engine.commands`, including UI commands `ui.tab`, `ui.dialog`, `ui.backstage`, `ui.zotero.*` (see `docs/zotero.md`), `ui.language` (`{"value": "auto"|"en"|"zh-hans"|"zh-hant"|"ja"|"pt-br"|"es"|"uk"}`), `ui.theme` (`{"value": "system"|"light"|"dark"}`; `system` follows the OS appearance, light when it reports none)…) |
 | `engine.commands` | — | every command: id, label, location, shortcut, params, enabled |
 | `document.inspect` | `text?` | blocks (text, style, runs, lists, tables), parts, sections, selection, pages |
 | `ui.inspect` | — | UI state, view, dialog, window size, page rects on screen, caret, perf |