Day and night themes in Android

AppCompat has a new theme called DayNight since Support Lib 23.2.0. So, if your activity's theme extends from one of the DayNight variants then you can switch between dark theme and light theme in your activity as follows.

int mode = isNight ? AppCompatDelegate.MODE_NIGHT_YES : AppCompatDelegate.MODE_NIGHT_NO;
getDelegate().setLocalNightMode(mode);
recreate();

As you can see, to take any effect for changing night mode the activity should be re-created.

@Override
public boolean onColorModeUpdated() {
    boolean isDarkMode = PdfViewCtrlSettingsManager.isDarkMode(this);
        if (isDeviceNightMode() != isDarkMode) {
            getDelegate().setLocalNightMode(isDarkMode
                ? AppCompatDelegate.MODE_NIGHT_YES
                : AppCompatDelegate.MODE_NIGHT_NO);
        recreate();
        return true;
    }
    return false;
}

public boolean isDeviceNightMode() {
    int currentNightMode = getResources().getConfiguration().uiMode
        & Configuration.UI_MODE_NIGHT_MASK;
    return currentNightMode == Configuration.UI_MODE_NIGHT_YES;
}

You can also have different styles for device day/night modes by creating new resources for night. This involves adding a new directory to the resources folder, using the night qualifier, to re-define the resources. In the following example, primary colors and accent color are different in day and night mode:

res/values/styles.xml

<style name="PDFTronAppTheme" parent="PDFTronAppThemeBase">
    <item name="colorPrimary">@color/color_primary_day</item>
    <item name="colorPrimaryDark">@color/color_primary_dark_day</item>
    <item name="colorAccent">@color/color_accent_day</item>
</style>

res/values-night/styles.xml

<style name="PDFTronAppTheme" parent="PDFTronAppThemeBase">
    <item name="colorPrimary">@color/color_primary_night</item>
    <item name="colorPrimaryDark">@color/color_primary_dark_night</item>
    <item name="colorAccent">@color/color_accent_night</item>
</style>