Hi there,
the Site you installed added CSS in Customizer > Additional CSS. To remove the white space border delete this CSS from there:
body {
margin: 5%; /* Space around site. */
position: relative;
}
This CSS also defines a max width of the content, you can also remove this if you don’t want that:
.no-sidebar .inside-article > *,
.no-sidebar #comments,
.no-sidebar .paging-navigation {
max-width: 700px;
margin-left: auto;
margin-right: auto;
}