Add the below to your CSS, beneath the jqGrid css:
.ui-corner-all, .ui-corner-top, .ui-corner-left, .ui-corner-tl, .ui-corner-right, .ui-corner-tr, .ui-corner-bottom, .ui-corner-bl, .ui-corner-br {
border-radius:0;
-moz-border-radius:0;
-webkit-border-radius:0;
-khtml-border-radius:0;
}