Add Dialog to change theme.

app/src/main/java/me/zobrist/tichucounter/MainActivity.kt

@@ -1,5 +1,6 @@
 package me.zobrist.tichucounter
 
+import android.app.AlertDialog
 import android.os.Bundle
 import android.view.Menu
 import android.view.MenuItem
@@ -8,6 +9,7 @@ import androidx.core.widget.doOnTextChanged
 import kotlinx.android.synthetic.main.content_main.*
 import android.text.InputType
 import android.widget.ScrollView
+import androidx.appcompat.app.AppCompatDelegate
 
 class MainActivity : AppCompatActivity() {
 
@@ -256,6 +258,10 @@ class MainActivity : AppCompatActivity() {
                 undoLastRound()
                 true
             }
+            R.id.action_theme -> {
+                chooseThemeDialog()
+                true
+            }
             else -> super.onOptionsItemSelected(item)
         }
     }
@@ -317,4 +323,44 @@ class MainActivity : AppCompatActivity() {
         submit.isEnabled = false
     }
 
+
+    private fun chooseThemeDialog() {
+
+        val builder = AlertDialog.Builder(this)
+        builder.setTitle(getString(R.string.choose_theme_text))
+        val styles = arrayOf("Light","Dark","System default")
+
+        var checkedItem = when(AppCompatDelegate.getDefaultNightMode()) {
+            AppCompatDelegate.MODE_NIGHT_NO -> 0
+            AppCompatDelegate.MODE_NIGHT_YES -> 1
+            AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM -> 2
+            else -> -1
+        }
+
+
+        builder.setSingleChoiceItems(styles, checkedItem) { dialog, which ->
+
+            when (which) {
+                0 -> {
+                    AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_NO)
+                    delegate.applyDayNight()
+                    dialog.dismiss()
+                }
+                1 -> {
+                    AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_YES)
+                    delegate.applyDayNight()
+
+                    dialog.dismiss()
+                }
+                2 -> {
+                    AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM)
+                    delegate.applyDayNight()
+                    dialog.dismiss()
+                }
+            }
+        }
+
+        val dialog = builder.create()
+        dialog.show()
+    }
 }