A premium theme doesn't give the option itself to customize beyond what is built in to the theme features. However, the custom design upgrade (a separate purchase) comes with a CSS editor you can use to make adjustments and customize the theme further if you'd like.
You have posted in the CSS Customization forum, so you may already have the Custom Design upgrade. Either way, you should be able to find the Appearance > Customize > CSS editor and add this so you can see an example in the preview window. For it to work, first you need to add something on one of the pages in a blockquote (indented paragraph) and then browse to where you can see it in the live preview. Once you have that, enter the following in the CSS editor to try it out:
border-top: 3px solid black;
border-bottom: 3px solid black;
You can adjust all the spacing and width values as needed.