Canvas: support transparent background colors for cards

[Sharknader27]

Btw, you can create transparent cards using CSS and apply that to “non-themed” (aka default grey) cards.

.canvas-node:not(.is-themed) .canvas-node-container {
   background-color: transparent; 
    border: 2px solid transparent;
} 

canvas-node-container {
    box-shadow:transparent;
}