choose_theme.kt

fun showThemeSwitcher(activity: Activity, prefs: SharedPreferences) {
    val builder = AlertDialog.Builder(activity)
    val names = mutableListOf<String>()
    var selectedTheme = getSelectedTheme(activity, prefs).ordinal
    builder.setTitle("Choose theme to apply")
    builder.setSingleChoiceItems(SwitchableThemes.values().mapTo(names) {
        it.label
    }.toTypedArray(), selectedTheme) { _, which ->
        selectedTheme = which
    }.setPositiveButton(R.string.ok) { dialog, which ->
        prefs.edit {
            putInt(activity.getString(R.string.debug_switch_theme), selectedTheme)
        }
        activity.recreate()
        dialog.dismiss()
    }.show()
}

get_theme.xml

fun getSelectedTheme(context: Context, prefs: SharedPreferences): SwitchableThemes {
    val themePos = prefs.getInt(context.getString(R.string.debug_switch_theme), 0)
    return SwitchableThemes.values()[themePos]
}

switchable_themes.kt

enum class SwitchableThemes(val label: String, @StyleRes val id: Int) {
    DEFAULT("WOW", R.style.Theme_Wow),
    REWARDS("Variant 1", R.style.Theme_Variant_1)
}