Fix issue #7633 by adding button style

Add `android:buttonBarButtonStyle` to `FallbackTheming.Dialog`, modifying the button style of the fallback `AlertDialog`, so that the button would not appear as invisible.

src/main/res/values/styles.xml

@@ -80,8 +80,15 @@
         <item name="windowNoTitle">false</item>
         <item name="android:windowBackground">@color/bg_default</item>
         <item name="android:textAllCaps">false</item>
+
+        <item name="android:buttonBarButtonStyle">@style/FallbackTheming.Dialog.ButtonStyle</item>
 	</style>
 
+    <style name="FallbackTheming.Dialog.ButtonStyle"
+        parent="Widget.MaterialComponents.Button.TextButton.Dialog">
+        <item name="android:textColor">#000000</item>
+    </style>
+
 	<!-- separate action bar style for activities without an action bar -->
     <style name="BaseTheme.ownCloud.Toolbar" parent="Theme.MaterialComponents.DayNight.NoActionBar">
         <item name="actionModeBackground">@color/action_mode_background</item>