diff --git a/index.html b/index.html
index 58f06d7..b3f7687 100644
--- a/index.html
+++ b/index.html
@@ -4,6 +4,16 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <link rel="icon" type="image/png" href="/margin-mark.png" />
+    <script>
+      (function () {
+        try {
+          var t = localStorage.getItem("margin-theme");
+          if (t !== "light" && t !== "dark")
+            t = matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
+          document.documentElement.setAttribute("data-theme", t);
+        } catch (e) {}
+      })();
+    </script>
     <title>margin</title>
   </head>
   <body>
diff --git a/src/components/EditorView.tsx b/src/components/EditorView.tsx
index 3c38e9e..b5173be 100644
--- a/src/components/EditorView.tsx
+++ b/src/components/EditorView.tsx
@@ -7,6 +7,7 @@ import { Settings } from "./Settings";
 import { Editor } from "../editor/Editor";
 import { FloatingToolbar } from "../editor/FloatingToolbar";
 import { useBook } from "../store/useBook";
+import { useTheme } from "../store/useTheme";
 import type { Book } from "../model/book";
 import { saveBook } from "../library";
 import { isDesktop } from "../ipc";
@@ -22,6 +23,8 @@ export function EditorView() {
   const setChapterContent = useBook((s) => s.setChapterContent);
   const setChapterTitle = useBook((s) => s.setChapterTitle);
   const markSaved = useBook((s) => s.markSaved);
+  const theme = useTheme((s) => s.theme);
+  const toggleTheme = useTheme((s) => s.toggle);
 
   const [editor, setEditor] = useState<TiptapEditor | null>(null);
   const [dock, setDock] = useState(true);
@@ -99,6 +102,16 @@ export function EditorView() {
               </>
             )}
           </div>
+          <button className="icon-btn" onClick={toggleTheme} title={theme === "dark" ? "Light mode" : "Dark mode"}>
+            {theme === "dark" ? (
+              <Icon>
+                <circle cx="12" cy="12" r="4" />
+                <path d="M12 2v2M12 20v2M2 12h2M20 12h2M4.9 4.9l1.4 1.4M17.7 17.7l1.4 1.4M19.1 4.9l-1.4 1.4M6.3 17.7l-1.4 1.4" />
+              </Icon>
+            ) : (
+              <Icon d="M21 12.8A9 9 0 1 1 11.2 3a7 7 0 0 0 9.8 9.8z" />
+            )}
+          </button>
           <button className="icon-btn" data-on={dock} onClick={() => setDock(!dock)} title="Toggle preview">
             <Icon d="M3 4.5h18v15H3zM14 4.5v15" />
           </button>
diff --git a/src/components/Icon.tsx b/src/components/Icon.tsx
index dbcd7ff..bc60a57 100644
--- a/src/components/Icon.tsx
+++ b/src/components/Icon.tsx
@@ -1,9 +1,12 @@
+import type { ReactNode } from "react";
+
 interface IconProps {
-  d: string;
+  d?: string;
   size?: number;
+  children?: ReactNode;
 }
 
-export function Icon({ d, size = 16 }: IconProps) {
+export function Icon({ d, size = 16, children }: IconProps) {
   return (
     <svg
       width={size}
@@ -15,7 +18,7 @@ export function Icon({ d, size = 16 }: IconProps) {
       strokeLinecap="round"
       strokeLinejoin="round"
     >
-      <path d={d} />
+      {children ?? <path d={d} />}
     </svg>
   );
 }
diff --git a/src/store/useTheme.ts b/src/store/useTheme.ts
new file mode 100644
index 0000000..e96818f
--- /dev/null
+++ b/src/store/useTheme.ts
@@ -0,0 +1,16 @@
+import { create } from "zustand";
+import { applyTheme, initialTheme, type Theme } from "../theme";
+
+interface ThemeState {
+  theme: Theme;
+  toggle: () => void;
+}
+
+export const useTheme = create<ThemeState>((set, get) => ({
+  theme: initialTheme(),
+  toggle: () => {
+    const next: Theme = get().theme === "dark" ? "light" : "dark";
+    applyTheme(next);
+    set({ theme: next });
+  },
+}));
diff --git a/src/theme.ts b/src/theme.ts
new file mode 100644
index 0000000..9f3eab0
--- /dev/null
+++ b/src/theme.ts
@@ -0,0 +1,16 @@
+export type Theme = "light" | "dark";
+
+const KEY = "margin-theme";
+
+export function initialTheme(): Theme {
+  const attr = document.documentElement.getAttribute("data-theme");
+  if (attr === "light" || attr === "dark") return attr;
+  const saved = localStorage.getItem(KEY);
+  if (saved === "light" || saved === "dark") return saved;
+  return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
+}
+
+export function applyTheme(theme: Theme) {
+  document.documentElement.setAttribute("data-theme", theme);
+  localStorage.setItem(KEY, theme);
+}