Adding colored borders to website

Hello,

I am trying to add colored borders to the css. sytle sheet of my website: newsite.nwsac.org

The theme (academica) hosted by wpzoom doesn’t allow for you to create borders on the side like these sites:
https://www.naturesemporium.com/
https://aroundthesoundfood.info/

But I can’t seem to get my borders to show up when I edit the header/footer/ and custom style sheets.

Any help? Im new to css coding