Gutenboarding: update design picker focus and hover style

[razvanpapadopol]

Suggestion for page selector, via #38583 (comment):

We can borrow from the existing starter page template selector styles - the suggestion below works for page selector cards. Still need to figure out how to work around a specificity collision with .is-elevated to use the same style on the design selector cards.

  &:focus {
  	box-shadow: 0 0 0 1px $white,
  				0 0 0 3px $blue-medium-500;
  	// Windows High Contrast mode will show this outline, but not the box-shadow.
  	outline: 2px solid transparent;
  }

  &:hover {
  	border: 3px solid var( --color-neutral-dark );
  	padding: 0;
  }