Fix bottom sheet background for PreviewImageFragment

Signed-off-by: Álvaro Brey <alvaro.brey@nextcloud.com>

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

@@ -256,6 +256,7 @@
         <item name="android:navigationBarColor">@color/black</item>
         <item name="toolbarStyle">@style/Theme.ownCloud.Overlay.ActionBar</item>
         <item name="actionOverflowButtonStyle">@style/ToolbarStyle.Overflow</item>
+        <item name="bottomSheetDialogTheme">@style/ThemeOverlay.App.BottomSheetDialog</item>
     </style>
 
     <style name="Theme.ownCloud.OverlayGrey" parent="Theme.ownCloud.OverlayBase">