Add a thin border to color palette blocks in the design system

Overview

• Adds a think semi-transparent border to colour blocks in the colour palette atom of the design system explorer.

Based on a suggestion from review feedback: !647 (comment 922848938).

How to Test

Start the design system explorer using

npm run live-design-system

And go to the Color Palette atom. All blocks should have a thin, light grey border to demarcate the bounds of the colour boxes (particularly) so that white-on-white boxes don't disappear into the page's background.

Screenshots