kcms/colors: Give preview cells a bit more space

Cell size is currently quite cramped for the scheme previews, to the point that its height overflows the card.

The height change is needed to fix this visual glitch, but while at it let's give it just a bit more width too. @teams/vdg

Also we don't need to set the implicit cell size imperatively. It was probably a workaround for a now solved issue.

BEFORE	AFTER