When you supply a theme color to a property, you're supplying a string name, and handing off handling of conversion to the browser in use, where the string name will eventually match a variable name and be converted into a hex value.
But in the Designer, any binding will only see the string value - there is no direct check ever made against the CSS files by Ignition because they're "downstream" of Ignition itself.
Oddly enough, there's no way to directly add alpha to a variable in CSS, but you can get the desired behavior by modify the opacity property of the component in question.
