Contact form7 is the most popular contact form plugin in WordPress. It is popular for it's functionality and also for it's user-friendly options.

You can use your custom design easily but if you want to use a custom design for the checkbox field it's a little bit complicated. From this post, you can to do this easily. See below the code for custom design checkbox field.

Code:

.wpcf7-checkbox span.wpcf7-list-item-label

.wpcf7-checkbox span.wpcf7-list-item-label:before

.wpcf7 input[type=checkbox]:not(old):checked + span:before
.wpcf7 input[type=checkbox]:not(old):checked + span:before
.wpcf7-checkbox span.wpcf7-list-item-label:after

.checkbox-item label:hover span.wpcf7-list-item-label:before

Contact forn7 Checkbox Design

Replace .checkbox-item from your any custom class and just change the color, height, width, border color, background color, position, and design checkbox field.

Enjoy!