What needs to go in the `.add-background-tint`-class in order to add a semi transparent background colour on top of the table stripes? I want to keep the stripey-ness, and highlight certain rows.

CSS:

```
.highlighted-row {
    /* what goes here? */
}

```

HTML:

```
<table class="table table-striped">
    <tr><td>row 1</td></tr>
    <tr class="highlighted-row"><td>row 2</td></tr>
    <tr class="highlighted-row"><td>row 3</td></tr>
    <tr><td>row 4</td></tr>
</table>

```