OK after A LOT of trial and error I worked it out. Not sure if there’s a more robust way but all I did was I added this code to ‘Edit CSS’. I suppose one could amend the max-width of the media to whatever one wanted, but the affects get a bit funky above a certain amount.
@media (max-width: 720px)
{
/* this edits screens w/o sidebar below 720px*/
.site-content article {
/* this reduces the white space between text the border of the page*/
padding-left: 10px;
padding-right: 10px;
}
#page {
/* remove the black border background*/
width: 100%;
}
}