In the HTML structure, GridClientSelectColumn renders <input type="checkbox"> elements in the table cells (<td> elements). Unfortunately the <input type="checkbox"> HTML element cannot be styled.
One option is to hide it by setting its opacity to 0 and place your own custom checkbox over, see How TO - Custom Checkbox article. Here is an article that I found that may also help: How to style checkbox without using any CSS framework.
To apply styles to the checkbox in the GridClientSelectColumn you can use the ItemCssClass property to assign a custom class to the rendered <td> element
I hope this will help.
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