I've installed IWD Onepage Checkout, and I'm trying to style it.

Rather than edit the 'core' installation, I want to add my own stylesheet -

I've added this to `/app/design/frontend/default/<theme>/layout/local.xml`

```
<opc_index_index>
    <action method="addCss">
        <stylesheet>css/custom-opc.css</stylesheet>
    </action>
</opc_index_index>

```

..however, my stylesheet isn't getting added?