It is possible to produce a white background region/box for the scale bar in the Layout. Once the scale bar is created, right-click on it and click on 'Frame Properties...' then click on the 'Style' button and adjust the region style to create the white background.
The end result would look something like this.
Currently I do not see any straightforward way to create the same effect for a north arrow in the layout window. This may need to be raised as an enhancement request. I can certainly see the value for it as if there is no background, then it may not be very clear. Perhaps someone from our Development team could comment on this?
Hope this helps!