add some styles to localgov_admin_theme_improvements to help with box links layouts

[finnlewis]

@finnlewis @markconroy in the drag and drop editor of a subsite overview these don't have any spacing between them in the edit window so the settings don't appear to make any difference until its rendered on the front end. I think we should add some styles to localgov_admin_theme_improvements to handle the case because it might confuse editors.

When I choose 'two per row' and I have three links it still displays three side by side in the editor, and I get some padding.

Originally posted by @tonypaulbarker in #887 (comment)

[markconroy]

Looks like the columns in the 3 column and other layouts do have spacing, but it's only 5px (which is our default in these layouts) until we get the "use grid for layouts" issue merged: #786

[markconroy]

I think we can close this issue.