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I assume you are referring to the matrix field? You can try using this custom CSS code and add it under the CSS tab in your form designer tool:
/**Check box unchecked**/
border : 2px solid black !important;
color : #444;
font : inherit;
margin : 0 1px 0 0;
outline : none !important;
text-shadow : 0 1px 0 #F0F0F0;
cursor : pointer;
width : 20px !important;
height : 20px !important;
/**When checkbox is selected**/
-webkit-appearance : checkbox; //Default check box when selected
...Add your own styling here
However, we can't design the default check marked, you might need to refer to an outside design or image and add it in the part where the check box is selected.
I hope that helps.