Light Switch

An Android app that flips your phone between the system light and dark theme with a wall light switch. Lever up = lights on (light theme), lever down = lights off (dark theme). Tap it, drag it, or flick it.

Built with Kotlin and Jetpack Compose. Requires Android 10 (API 29) or newer, which is when Android gained a system-wide dark theme.

Home screen widget

Long-press your home screen, choose Widgets, and add Light Switch. The widget is a tap-to-flip switch: lever up means light theme, lever down means dark. It follows the system theme, and before the one-time setup below is done a tap opens the app instead.

One-time setup

Android does not let ordinary apps change the system theme. The app needs the WRITE_SECURE_SETTINGS permission, which only a shell (ADB) can grant. You do this once; it survives reboots and app updates (but not an uninstall).

Option A: from the phone, no computer (Android 11+)

1. Install Shizuku from Google Play and open it.
2. Tap Start via Wireless debugging and follow Shizuku's on-screen steps (turn on Developer options, enable Wireless debugging, pair, start).
3. Open Light Switch and tap Grant permission on the setup card. Approve the Shizuku prompt. Done; the switch works and Shizuku is no longer needed.

Option B: from a computer

1. Enable Developer options and USB debugging on the phone.

2. Connect the phone and run in a terminal:

   adb shell pm grant com.samhrncir.lightswitch android.permission.WRITE_SECURE_SETTINGS

3. Reopen the app. The setup card disappears and the switch works.

The app shows both options, with a Copy button for the command, until the permission is granted.

Installing on your phone

Every push builds the app on GitHub Actions and publishes the APK under Releases → Latest build (also as a workflow artifact).

1. On the phone, open the repository's Releases page and download LightSwitch.apk.
2. Open the downloaded file. Allow installs from your browser if asked.
3. Complete the ADB step above once.

Builds are signed with the debug key in app/debug.keystore, so newer APKs install over older ones without uninstalling.

Building yourself

Open the project in Android Studio (Ladybug or newer) and run it, or from the command line with the Android SDK installed:

./gradlew :app:installDebug

How it works

• ThemeController writes the secure setting ui_night_mode (1 = light, 2 = dark). Android's UI mode service only re-reads that setting when leaving car mode, so the app enters and immediately exits car mode to apply the change at once. This is the same technique automation apps use. You may notice the car-mode notification flash for a split second.
• On builds that do not lock day/night mode (for example Android Automotive), UiModeManager.setNightMode works directly and is tried first.
• LightSwitch is a Canvas-drawn composable. It animates the lever as soon as you interact, asks the controller to change the theme, and springs back if the system never confirms the change.
• ShizukuHelper asks Shizuku, when the user has started it, to run pm grant for this app so the permission can be granted from the phone itself.
• MainActivity handles uiMode configuration changes itself so the switch animation is not interrupted when the theme flips.