In order to change this, you need to open the Theme Object defined in the property Default Theme of your KB in Preferences. Once opened, look in the folder Customs for the class DIV.gx-mask:

There you can edit the class properties on the right pane tab Properties, for example:

If you need to add !important properties, go to the last property "Custom properties" and there you can add:

background-color: rgba(128, 128, 128, 0.5) !important; opacity: 1 !important;