Unfortunately, it can’t be changed through css.
When you are using “lock” feature, all fields in the form are disabled, and according to HTML spec, a text of disabled input can’t be selected.
So the html spec says that a disabled field cannot be selected for copy and paste and Chrome is just getting around to adhering to the spec? Makes sense.
There is no such method in the dhtmlxForm.
If the situation is critical for you you may post to sales[at]dhtmlx.com to get the details about the customization possibility.