fix: checkbox style in black theme

In the black theme, the checkbox had a black tint
This had no contrast with the background, so change it
to white

Fixes 15638

AnkiDroid/src/main/res/values/styles.xml

@@ -195,6 +195,7 @@
 
     <style name="AlertDialogStyle" parent="Theme.AppCompat.Dialog.Alert">
         <item name="colorPrimary">?attr/progressDialogButtonTextColor</item>
+        <item name="buttonTint">?attr/colorOnPrimary</item>
         <item name="dialogCornerRadius">@dimen/dialog_corner_radius</item>
         <item name="android:background">?attr/dialogBackground</item>
         <item name="android:dialogCornerRadius" tools:targetApi="p">@dimen/dialog_corner_radius</item>