You have CheckboxSelection model which have defines:
PHP Code:
width 42,
header '<div class="x-grid3-hd-checker" style="width: 20px; margin-left: 20px;"> </div>'
However, there is still old checkbox as background-image, because <td class="x-grid3-hd x-grid3-cell x-grid3-td-checker x-grid3-cell-first"> has inside <div class="x-grid3-hd-inner x-grid3-hd-checker">. Last div has inside header template from config.