[FEATURE] New display group variant - neutral grey

[jfmcquade]

What? A new variant/style for the display group that looks like the box in the attached image

Why? See issue on content repo. This style would likely have other general applications beyond the specified page.

How? This visual style should be a new variant (see #1971 for info on "variants")

[esmeetewinkel]

Might make sense to expose a couple of variants to authors, e.g.

• white / grey / primary / secondary background colour
• border dashed / solid / none
• border colour?