[Bug] App theme stays Light when system theme is set to automatically switch between Dark and Light themes

Describe the bug

When I set the system theme to be dark from sunset to sunrise, and set the DDG theme to system default, the app remains in light theme all the time.

How to Reproduce

1. Set your phone’s dark mode to be automatically enabled from sunset to sunrise.
2. Open DDG app.
3. Go to Settings.
4. Set Theme option to “System Default”.
5. Observe that the app always stays in light mode, even during the night.

Expected behavior

The app should be in dark mode from sunset to sunrise in your geographical region, and in light mode from sunrise to sunset. All other apps behave accordingly (Firefox, Bitwarden, Aegis, and Signal are few OSS app examples that behave correctly, might be useful when investigating the root cause of the problem)

Environment

- DDG App Version: 5.111.0
- Device: OnePlus 7T Pro
- OS: Android 11

[simonvar]

I think there is need just small fix in Theming.kt:

fun AppCompatActivity.applyTheme(theme: DuckDuckGoTheme): BroadcastReceiver? {
    if (theme == DuckDuckGoTheme.SYSTEM_DEFAULT) {
        val themeId =
            when (resources.configuration.uiMode and Configuration.UI_MODE_NIGHT_MASK) {
                Configuration.UI_MODE_NIGHT_YES -> THEME_MAP[Pair(manifestThemeId(), DuckDuckGoTheme.DARK)]
                Configuration.UI_MODE_NIGHT_NO -> THEME_MAP[Pair(manifestThemeId(), DuckDuckGoTheme.LIGHT)]
                else -> THEME_MAP[Pair(manifestThemeId(), DuckDuckGoTheme.LIGHT)]
            }
        if (themeId != null) setTheme(themeId)
    } 
    ....
}

And, by the way, in DuckDuckGoActivity there is fun isDarkThemeEnabled(): Boolean which duplicate wrong logic from applyTheme and not in use.

[simonvar]

@malmstein cool theme refactoring, but please fix auto night mode, also this bug now in getThemedDrawable too. UiModeManager provides only static information about night mode (yes/no/auto/custom), needs to get dynamic info about mode from Configuration. I’ll provide PR with fix.