An example of the RGBa syntax is as follows:

.modal_background {
    background: rgba(0, 0, 0, 0.5);
}

The first three values are the colors and the last is the alpha value. Its on a scale of 0-1 and accepts decimals such as 0.5 or 0.55.

Browser Support:

Blackberry: BBOS 6+
Android: 2.1 +
iOS: 3.2 +
IE: 9+
Other Desktop: Everything

As you can see almost any current browser with the exception of IE supports RGBa color values. This means we can use them without worrying about compatibility.

Here’s a little preview of some of the things you can do!

Heres the code for the examples above:

.dark{
    background: rgba(0, 0, 0, 0.75);
}
.lighter{
    background: rgba(0, 0, 0, 0.5);
}
. lighest {
    background: rgba(0, 0, 0, 0.25);
}

Add a comment if you have questions!