Note: both of the snippets above will style all radio and checkbox elements on your site, so you'll probably want to add a more specific class to that to target specific forms.
Change the checkbox styling
Another thing you may want to do is set a different styling to your checkbox. Because you can't style checkboxes this is usually done by hiding the actual checkbox and adding in a fake one using a span in the markup (see an example here).
In HubSpot the radio button markup looks like this (I've stripped it down for simplicity):