Use a specific design for the "Edit field parameters" fieldset

When editing the settings related to a field instance, the form contains 2 field-sets:
the first one that exposes the instance settings
the last one that exposes the field settings. Those settings are global and apply to all instances of the same field.

It would be very useful to have a specific design for the second kind of settings, because modifying them may have impact to other instances and cause data loss.

The ID of the field-set is edit-field.

The specific design for this field-set may be a different background, or some borders / indents...