Using non-themed the card will still have a background color and I cannot find the CSS class for this.
Instead, I change the first color in the color picker to a transparent color. The CSS snippet is attached below:
.canvas-wrapper .canvas-node.is-themed.mod-canvas-color-1 .canvas-node-container {
background-color: transparent;
border: 2px solid transparent; /*Change this if you want to have border*/
}
.mod-canvas-color-1 {
--canvas-color: transparent;
}
/*Hide file name*/
.canvas-node.is-themed.mod-canvas-color-1 .canvas-node-label {
display: none;
}