Handle serialize for checkboxes with image and linear gradient

From #5

style.css has (not working version is commented out)

```css
/* CHECKED CARD */

.card .cb-container .cb-input:checked + .check {
/* background: url("../assets/images/icon-check.svg"),
linear-gradient(45deg, var(--clr-green), var(--clr-pink)); */
background: url('../assets/images/icon-check.svg');
background-color: var(--clr-green);
background-repeat: no-repeat;
background-position: center;
}
```

Start by inspecting style.css and the commented checked-card rule shown in the issue. Determine how the checkbox image and linear gradient are expected to be serialized, then verify that the checked card retains both visual elements.

Written by the indexing model from the issue text.