All CSS styles which uses jqGrid for the add/edit forms you can find here and here. If you would examine the lines and examine the HTML fragment of any jqGrid form you would see all classes used by jqGrid.
I don't understand only what you mean under "to import another css". The "import" would works only if you have somewhere another CSS styles with the same class names and the same hierarchy of the elements (dives, tables, tr and so on elements). So it's possible to change the styles of forms used by jqGrid, but the adjustment of the styles will be not so easy. You have to examine the structure of the jqGrid dialogs (forms) exactly to be able to make the changes.
UPDATED: jqGrid uses jQuery UI styles. So you need just change the jQuery UI to another one and jqGrid will use it.
For example the demo (simple modification of the old demo from the answer) produce the following Edit form:
