All the display elements can be modify in style resides in CSS folder. To adjust text-align, find the CSS file of your theme used. The default theme is "start’.

Look for:

ui-th-column,.ui-jqgrid .ui-jqgrid-htable th.ui-th-column
{overflow:hidden;white-space:nowrap;text-align:center; …
And change the text-align.