Currently, the CSS styles do not align the checkbox label with the checkbox. The label alignment is below the checkbox.

The fix is simple. The current CSS styling is this:

form div.marketing-customfield input[type="checkbox"],
form div.marketing-field input[type="checkbox"],
form div.marketing-subscription-list input {
width: 20px;
height: 20px;
margin-right: 8px;
margin-top: 3px;
}

Simply adding these three lines corrects the alignment:

vertical-align: middle;
position: relative;
bottom: 1px;

The new styling is this:
form div.marketing-customfield input[type="checkbox"],
form div.marketing-field input[type="checkbox"],
form div.marketing-subscription-list input {
width: 20px;
height: 20px;
margin-right: 8px;
margin-top: 3px;
vertical-align: middle;
position: relative;
bottom: 1px;
}