Changing Border Location When Stacking Columns Responsively

I have a 2 column design. The left column (the quote) has a detailed border width on the right of 2 px to separate it from the left column (floating image with text)

When viewed in a responsive layout the border looks fairly silly on the right and I have nothing separating the 2 columns when they are stacked on top of each other.

What could I do to keep the desktop layout but add a separation when stacked?

I am using Elixir’s Rustic theme and structure stack.