Introduce "Custom styles / Templates" section in core cards for much easier change of CSS/styling of cards (like bubble card)

[nagtegaal]

Introduce a "Styles" section in all the default available core cards to lower the bar for customizations and easier editing of CSS.
A magnificent example could be found in the amazing bubble-card.

This will eliminate the need for the excellent, but hard to grasp card_mod which is so widely used to style front-end in-card and card elements differently.