Our CSS changes (so far) In case it’s helpful to someone else:
// remove the grey background
.d-editor #form-template-form {
background-color: var(--secondary);
}
// stop stretching the fields to 100% width
.form-template-field select, .form-template-field input {
width: auto;
}
.form-template-field textarea {
min-width: 650px;
width: auto;
}