To ease the troubleshooting, can you add the CSS to the Custom CSS area directly?
Alternatively, try adding !important to the style properties like this:
.spjs-form-choice-wrap .spjs_chk_checkmark,
.spjs_radio_wrap .spjs_radio_checkmark {
background-color: white!important;
}
.spjs_chk_wrap input:checked~.spjs_chk_checkmark,
.spjs_radio_wrap input:checked~.spjs_radio_checkmark {
background-color: gray!important;
}
Alexander