fix(tui): detect Windows system dark mode via registry

packages/tui/src/context/theme.tsx

@@ -19,6 +19,7 @@ import {
   upsertTheme,
   type ThemeJson,
 } from "../theme"
+import { win32SystemTheme } from "../terminal-win32"
 import { createEffect, createMemo, onCleanup, onMount } from "solid-js"
 import { createStore, produce } from "solid-js/store"
 import { createSimpleContext } from "./helper"
@@ -160,7 +161,15 @@ export const { use: useTheme, provider: ThemeProvider } = createSimpleContext({
             if (store.active === "system") setStore("active", DEFAULT_THEME)
             return
           }
-          const next = store.lock ?? terminalMode(colors) ?? mode
+          // On Windows Terminal the buffer background (OSC 11) is independent
+          // of the system dark/light mode.  The title bar follows the system
+          // theme but the background colour does not change.  Use the Windows
+          // registry value as the authoritative source when available.
+          let next = store.lock ?? terminalMode(colors) ?? mode
+          if (process.platform === "win32") {
+            const winMode = win32SystemTheme()
+            if (winMode) next = winMode
+          }
           if (store.mode !== next) setStore("mode", next)
           const signature = JSON.stringify(colors)
           hasResolvedSystemTheme = true
@@ -243,12 +252,26 @@ export const { use: useTheme, provider: ThemeProvider } = createSimpleContext({
     let unsubscribeRefresh: (() => void) | undefined
     unsubscribeRefresh = themes.subscribeRefresh?.(refresh)
 
+    let winThemeWatcher: ReturnType<typeof setInterval> | undefined
+    if (process.platform === "win32") {
+      let lastWinTheme = win32SystemTheme()
+      winThemeWatcher = setInterval(() => {
+        const current = win32SystemTheme()
+        if (current && current !== lastWinTheme) {
+          lastWinTheme = current
+          if (!store.lock) apply(current)
+        }
+      }, 3000)
+      winThemeWatcher.unref()
+    }
+
     onCleanup(() => {
       renderer.off(CliRenderEvents.THEME_MODE, handle)
       renderer.removeInputHandler(handleThemeNotification)
       unsubscribeRefresh?.()
       for (const timeout of themeRefreshTimeouts) clearTimeout(timeout)
       themeRefreshTimeouts.length = 0
+      if (winThemeWatcher) clearInterval(winThemeWatcher)
     })
 
     const values = createMemo(() => {

packages/tui/src/terminal-win32.ts

@@ -128,3 +128,46 @@ export function win32InstallCtrlCGuard() {
 
   return unhook
 }
+
+/**
+ * Detect Windows system dark/light mode from the registry.
+ *
+ * Reads `AppsUseLightTheme` from:
+ *   HKCU\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize
+ *
+ * Returns `"dark"` when the value is `0`, `"light"` when `1`, or `undefined`
+ * if the query fails (registry key absent on older Windows, non-Windows platform).
+ */
+/**
+ * Parse the `AppsUseLightTheme` DWORD out of `reg query` output. Anchored to the
+ * value name so an unrelated hex token elsewhere in the output can't be picked up.
+ * `0` means dark mode, anything else light.
+ */
+export function parseWin32Theme(output: string): "dark" | "light" | undefined {
+  const match = output.match(/AppsUseLightTheme\s+REG_DWORD\s+0x([0-9a-f]+)/i)
+  if (!match) return undefined
+  return parseInt(match[1]!, 16) === 0 ? "dark" : "light"
+}
+
+export function win32SystemTheme(): "dark" | "light" | undefined {
+  if (process.platform !== "win32") return undefined
+  if (typeof Bun === "undefined") return undefined
+  try {
+    // timeout: reg.exe can hang (AV interception, contested handle) and this runs
+    // synchronously on the render thread. windowsHide: avoid a console flash.
+    const result = Bun.spawnSync(
+      [
+        "reg",
+        "query",
+        "HKCU\\Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
+        "/v",
+        "AppsUseLightTheme",
+      ],
+      { timeout: 1500, windowsHide: true },
+    )
+    return parseWin32Theme(result.stdout?.toString() ?? "")
+  } catch {
+    // reg.exe missing (older Windows) or spawn failure — treat as unknown.
+  }
+  return undefined
+}

packages/tui/test/win32-theme.test.ts

@@ -0,0 +1,31 @@
+import { expect, test } from "bun:test"
+import { parseWin32Theme, win32SystemTheme } from "../src/terminal-win32"
+
+test("win32SystemTheme returns a valid dark/light value on Windows, undefined elsewhere", () => {
+  const result = win32SystemTheme()
+  if (process.platform === "win32") {
+    // On Windows the registry may or may not be readable; both outcomes are valid.
+    // If readable, it must be "dark" or "light".
+    expect(["dark", "light", undefined]).toContain(result)
+  } else {
+    expect(result).toBeUndefined()
+  }
+})
+
+test("parseWin32Theme parses AppsUseLightTheme DWORD from reg output", () => {
+  const key = "HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize"
+  expect(parseWin32Theme(`${key}\r\n    AppsUseLightTheme    REG_DWORD    0x0\r\n`)).toBe("dark")
+  expect(parseWin32Theme(`${key}\r\n    AppsUseLightTheme    REG_DWORD    0x1\r\n`)).toBe("light")
+  expect(parseWin32Theme("    AppsUseLightTheme    REG_DWORD    0x00000000")).toBe("dark")
+  expect(parseWin32Theme("    AppsUseLightTheme    REG_DWORD    0x00000001")).toBe("light")
+  expect(parseWin32Theme("no match here")).toBeUndefined()
+  expect(parseWin32Theme("")).toBeUndefined()
+  // Anchored: a stray hex token that is NOT the AppsUseLightTheme value is ignored.
+  expect(parseWin32Theme("SomeOtherValue    REG_DWORD    0x1\r\n(no AppsUseLightTheme line)")).toBeUndefined()
+})
+
+test("win32SystemTheme is idempotent - second call returns same value as first", () => {
+  const a = win32SystemTheme()
+  const b = win32SystemTheme()
+  expect(a).toBe(b)
+})