Learn how dark themes work, first-hand information is Official Documents Corresponding Google Sample

Official document: DayNight - Adding a dark theme to your app:
https://medium.com/androiddevelopers/appcompat-v23-2-daynight-d10f90c83e94
Official document: Dark theme:
https://developer.android.com/preview/features/darktheme
Official case: android-DarkTheme
https://github.com/googlearchive/android-DarkTheme

1. Introduction to Dark Theme

Starting with Support Library 23.2.0, AppCompat has added a new theme: Theme.AppCompat.DayNight

• It allows APP to switch between dark and bright themes
• Can significantly reduce power consumption (On devices with OLED displays, dark themes have longer durability than bright themes)
• Improve amblyopia and visibility to light-sensitive users
• Enhance the user experience by making it easier for everyone to use the device in a dark environment

Starting with Android Q (10.0), a dark theme background toggle button (Setup-Display-Dark theme background) has been added to the Android settings.

So Android App supports nighttime mode and needs to be on the development agenda...

2. How to Use

2.1 Theme Settings

The APP theme inherits Theme.AppCompat.DayNight or Theme.MaterialComponents.DayNight, as shown in the code below.

<!-- Base application theme. -->
<style name="AppTheme" parent="Theme.AppCompat.DayNight">
    <!-- Customize your theme here. -->
</style>

2.2 Monitor Android Q System Theme Changes

If you need to listen to system themes, such as Settings-Display-Dark Theme Background Switching.

Activity adds android:configChanges="uiMode"

<activity
        android:name=".MyActivity"
        android:configChanges="uiMode" />

Override onConfigurationChanged method in Activity

/**
 * Android Callback this method after switching Settings-Display-Dark Theme Background in System Settings
 */
Override
public void onConfigurationChanged(@NonNull Configuration newConfig) {
    super.onConfigurationChanged(newConfig);
    int mSysThemeConfig = newConfig.uiMode & Configuration.UI_MODE_NIGHT_MASK;
    switch (mSysThemeConfig) {
        // Bright theme
        case Configuration.UI_MODE_NIGHT_NO:
            break;
        // Dark Theme
        case Configuration.UI_MODE_NIGHT_YES:
            break;
    }
}

2.3 Switch App Themes

To switch application themes in App, you first need to call the AppCompatDelegate.setDefaultNightMode(int mode) method and the recreate() method to make the changes take effect.

// Switch to Dark Theme
AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_YES);
// recreate() needs to be called for the changes to take effect
recreate();

The AppCompatDelegate.setDefaultNightMode(int mode) method has four parameter options, which are described below:

// Bright theme
ThemeHelper.Mode.LIGHT
// Dark theme
ThemeHelper.Mode.DARK
// Follow system settings (system dark mode, dark mode; system light mode, light mode)
AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM
// Dark mode in power saving mode; Light mode in non-power saving mode
AppCompatDelegate.MODE_NIGHT_AUTO_BATTERY

2.4 Monitor App Theme Changes

If you need to listen for App theme changes, you can override the following

/**
 * Callback to the current application's usage theme
 */
@Override
protected void onNightModeChanged(int mode) {
    super.onNightModeChanged(mode);
    switch (mode) {
            // Bright theme
        case AppCompatDelegate.MODE_NIGHT_NO:
            break;
            // Dark theme
        case AppCompatDelegate.MODE_NIGHT_YES:
            break;
            // Dark mode in power saving mode; Light mode in non-power saving mode
        case AppCompatDelegate.MODE_NIGHT_AUTO_BATTERY:
            break;
            // Follow system settings (system dark mode, dark mode; system light mode, light mode)
        case AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM:
            break;
    }
}

2.5 Custom Background Color

• New values-night folder

• Copy values/styles.xml to values-night/styles.xml

Examples of values/styles.xml and values-night/styles.xml theme codes are as follows

values/styles.xml

<resources>
    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.AppCompat.DayNight">
        <!-- Customize your theme here. -->
    </style>
</resources>

values-night/styles.xml

<resources>
    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.AppCompat.Light">
        <!-- Customize your theme here. -->
    </style>
</resources>

• Copy values/colors.xml to values-night/colors.xml

Change the color value of the test_text_bg property

values/colors.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="colorPrimary">@android:color/white</color>
    <color name="test_layout_bg">@android:color/white</color>
    <color name="test_text">@android:color/black</color>
    <color name="test_text_bg">#008577</color>
</resources>

values-night/colors.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="colorPrimary">@android:color/black</color>
    <color name="test_layout_bg">@android:color/black</color>
    <color name="test_text">@android:color/white</color>
    <color name="test_text_bg">#D81B60</color>
</resources>

3. OK is done

Case Source Download Address:
https://github.com/AndroidAppCodeDemo/Android_Dark_Test