Moving this question to AEM Forms.
To change the css of radio/checkbox component , first select that component in theme editor in edit mode. It should look like below screenshot
In left side of screen you can see the options to change the css of checkbox. Select the state of checkbox as selected and apply the changes you want to do for selected state.
In your case you can give background-color or shadow-color to checkbox widget in selected state.
Hope it helps !!
I am not able to override the default image comes for the checkbox. Using the methods mentioned above its filling the sadhows but on click i need to show a checkbox with blue backgrounds and white colored tick mark. if I am giving my new image as background in selected stated then its coming in the backside of the defalut checbox icon. Can I move it to the front some how?