Behind the Scene
I tried to change the CSS of jqgrid and match it with the theme of CRM 2013, but found some problem with it. Then we have to remove the CSS used for the Grid and build a new CSS file for 2013.
The CSS, which were removed from the HTML page :-
ui.jqgrid.css (given in the jqGrid Plugin folder)
The CSS newly added for CRM 2013 is :-
ui.jqgrid_custom.css (Custom build)
Feel free to click and download the CSS file and implement in your CRM projects.