Modals are made from Popup pages. Below is a x-ray 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.