Modals are made from Popup pages. Below is a breakdown of the various CSS classes that make up a modal.

Styling the Backdrop

You can change the color of the modal backdrop with the CSS below:

^popuppage {
    -rock-modal-backdrop-color: #80000000;
}

The color value #80000000 will set the backdrop color to black with a 50% alpha transparency.

Here is a cheat sheet of various alpha to Hex Colors.