Checkbox styling in Advanced Grid widget

Thingworx 8.5.2 :

As I see,

Checkboxes inside advanced grids are made using images.So,I tried to change images as per required design as attached in the document but I couldn't achieve as it is not allowing the class name.Please let me know the class I should use in custom css area.

Class which I am trying to use is not allowing  selectors

.widget-gridadvanced .objbox table tbody tr [src*="item_chk0.gif"]

{

background-image : url(.....);

}