This is an example page with a top border (instead of the default Full Border)
It is very easy to change the page border style, simply go to the page you wish to change, click Edit Page, and change the style in the sidebar meta area:
You can also change the overall default top border style from the Theme Customize area. Just go to Appearance > Customize > Page Style and choose the defaults there.
These defaults will apply to all pages, and you can change it on a per-page basis as above.